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Keep a Brushless ESC and Motor cool by gearing properly.

To keeping the motor from overheating must be looked at on both sides, under and over gearing.

1- don't over gear (tall or high gearing), meaning: If your pinion is too large and/or your spur gear is too small then it will be over geared. To get the motor to maintain the desired speed it takes more torque Current. The motor may not be able reach the desired speed as the batteries and esc, and the motor windings themselves can only handle so much current before becoming damaged.

2- don't under gear (short or low gearing), meaning: If your pinion is too small and/or your spur gear is too large then it will be under geared. This means your motor reaches maximum rpm too quickly because of applying higher Voltage, effectively placing too small of a load on the motor- running a motor unloaded ( no real resistance ) can also damage the rotor over time.

Volts multiplied by Amps equals Watts, More Watts equals more Heat.

Motor efficiency plays a huge factor in this. The better the components such as wire quality, insulation quality, precision windings, bearing quality, magnet quality.... the more efficient the motor will be. Better costs more and you get what you pay for.
While there is no 100% efficient motor, there is a huge difference between a 70% efficient and an 85% efficient motor. If you look at 7.4V * 50A you get 370W. On the 70% motor 30% turns directly to heat, 111watts. Verses the 15% from the 85% motor for 55.5watts of heat. That little difference in quality results in a huge 50% difference in inefficient heat. This much difference is very common between cheap motors and the quality motors on the market today. Likewise about the components in the ESC.

There is a lot to consider about your driving style and terrain.

It is not as simple as picking a motor and gearing, it is picking the right motor and gearing for the conditions. What terrain, what battery, what driving style? There are other things to consider as well when you involve aftermarket parts such as tires and wheels of different size and weight, excessive weight from aftermarket alloy parts, and other changes. A
Frequent slowing and stopping with quick acceleration will require more low-end power or lower gearing. Usually short or technical tracks need lower gearing. Likewise rougher terrain such as running in grass also require lower gearing. Care must be taken because you will not have the higher speed of taller gearing and it is easy to push your truck hard and over heat your ESC and/or Motor.
Long stretches of track, oval tracks, or open areas will require higher gearing to get higher speeds. Going too tall on the gearing will over stress the motor and/or ESC which will cause over heating.
Testing by trial and error is the best way to find optimal gearing. Everyone drives differently even on the same terrain.
Remember that changing from one style driving and/or terrain to another will many times require re-gearing to obtain optimal performance.
Also remember that changing to different battery packs (increased voltage or from NiMH to LiPo) will also many times require re-gearing for optimal performance. Driving in grass puts a lot of strain on your motor and ESC no matter what the gearing, don't push it too hard.

What does changing the Pinion Gear (gear on the motor) do?

Without changing the Spur Gear or Motor RPM, smaller Pinions (or lower gearing) will reduce the driving speed and increase torque. Opposite, larger will increase the speed and decrease the torque.

What does changing the Spur Gear (gear on the transmission input shaft) do?

Without changing the Pinion Gear or Motor RPM, Smaller Spurs will increase the driving speed and reduce torque. Larger will decrease the speed and raise torque.
As you can see changing the pinion and spur work in opposite direction for speed changes.
Remember it is possible to change both the pinion and spur and still have the same gear ratio. Example 10/25 gearing is equal to 20/50 gearing. Both are a ratio of 2:5

Brushless Motors "KV" ratings (rpm per volt) determined by the number of wire "turns" in the motor.

KV is not the same thing as "kV" or Kilo-Volt. The higher the KV rating on a motor the faster it turns with the same applied voltage as a lower rated motor.
The lower the Turn count the faster the motor turns at the same applied voltage as a higher count motor.
If your motor is too slow or too fast, you may not be physically able to find or fit the correct size gears to operate in the optimal range. Most people use 540 size brushless motors in the 3000KV to 6000KV range or 6.5 to 13.5 turn. It is possible to use motors out of that range.

Temperature

Generally speaking, With a Temp gun a warm/hot motor & ESC should be around 150° to 170° Fahrenheit and under 200° Fahrenheit. Some motors and ESCs are different depending upon construction, check the manufacturer specifications.

Example:
Castle Creations says " As long as the motor is less than 200 degrees F (would burn your finger almost instantly) at the end of the run, you’re OK."

Novak says "the maximum external safe operating temperature for Novak brushless motors is 175 degrees F (80 degrees C). Motors that run at higher temperatures have sustained internal damage. From our testing and experience, keeping the external motor operating temperature under 160 degrees F (72 degrees C) will provide the best performance when the car is handling well.

Since most people don't use a temp gun, here is an easier way to check.

-WARNING- This could be dangerous, take care in using the following technique. Children should not use this technique.
You should be able to put your finger on the the can of the motor or fins of the ESC and not feel pain. This is still subjective since everyone has a different threshold of pain, I would guess mine to be average to slightly above average for middle aged people in this hobby.

Cold
-doesn't feel any different than before driving.

Cool
-You can tell it is warmer than before driving but not much warmer than that.

Warm
- Noticeably different than Cool but you can hold your finger on it for any length of time.

Very warm
-You can keep your finger on the motor for a long period of time but it is not comfortable. This is optimal for general use.

Hot
-You can put your finger on it but you need to remove it because it starts to hurt. You need to consider changes. You are beginning to push the truck too hard. This is where I try to stop pushing my trucks.

Where I draw the danger line, you should not run a motor hotter than this or about 200 degrees F.

Very Hot
-You can touch your finger to it but need to remove it quickly because you it is very uncomfortable and would become painful very quickly, your instinct is to pull your hand back. This is where thermal breakdown becomes destructive to your electronics and you should take action to prevent more heat. Shut your stuff off, let it cool, and find a solution. There IS a Problem even if it is you expecting too much out of the truck.

Super Hot!
-You can barely touch it and your feel immediate pain, your immediate instinct is to pull your hand back.
NEVER let your motor get this hot!

Super Freaking HOT!!!
-Severe pain and you have to remove your finger immediately and blisters form, your instinct doesn't matter because your natural reaction caused every muscle in your arm to contract to pull your hand to safety. At this point, you've probably severely damaged something and your motor & ESC are likely damaged too.

Specifically for the HPI Blitz
There is a basic gearing guide in the Blitz manual on page 18.
Stock Gearing is 18t pinion and 88t spur (marked STD. on the chart). You will find an 83t spur in the extra parts bag included with a new Blitz.
CLICK HERE for the Blitz online PDF Manual.
This will get you close to optimal.

This is my opinion and approach to keep the ESC and Motor cool by gearing properly.