If your vehicle is showing transmission fluid on dipstick looks black or smells burnt, here are the most likely causes and the parts you'll need to fix each one.
Transmission fluid naturally degrades, oxidizes, and darkens over time. If the fluid has exceeded its service interval, it will turn black and develop a slightly burnt odor simply from prolonged normal heat cycles.
Heavy towing, hauling, or aggressive driving without adequate cooling causes the fluid to overheat. Extreme temperatures rapidly break down the fluid, causing it to literally burn and turn black.
A fluid leak or an improper fill level during a recent service reduces hydraulic pressure. This causes the internal clutches to slip, generating massive amounts of friction and heat that cooks the remaining fluid.
The friction material on the transmission's internal clutches has worn away or burned up. This sheds black, sooty debris directly into the fluid and causes severe slipping.
The internal lock-up clutch inside the torque converter is failing and slipping. This acts as a massive heat generator, rapidly burning the fluid before circulating it back into the transmission.