If you find that your car has black transmission fluid, it means you have probably not changed it in a long time. Used transmission fluid should be red or brown in color. If the fluid is black, it is past its useful time frame and needs to be replaced immediately. Your transmission may need to also be flushed to get rid of any debris or particles that may be in the lines. If it is black, it means the fluid has been burnt, and is probably causing slippage in your transmission function. Change your transmission fluid more often and this won’t happen to your car or truck.