Most likely it is the engine oil leaking into the front part of the transmission. There is a seal there -- the rear main seal -- that seals the end of the crankshaft where it exits the engine and connects to the flywheel or flexplate to transmit the engine power to the transmission. Engine oil is black in color, and automatic Transmission Fluid is largely red in color. Any dirty fluid can appear black or blackened, so it is good to clean all areas where the fluid is leaking, and then use a white paper towel to collect some of the freshly leaking fluid to determine its color.
There are several indicators of problems with a cars transmission. Examples of these include lack of response while driving and leaking transmission fluid.
The better question is what is leaking transmission fluid?
Because your transmission has a seal that is leaking.
The automatic transmission has a vent tube. If the transmission is overfilled or gets water in it , the transmission may appear to be leaking tranny fluid after the transmission has warmed up from driving because the tranny fluid expands.
The output shaft seal has failed and transmission fluid is leaking by, and that would explain the transmission fluid appearing there.
Slippage is oil leaking from a pressure outlet to a low-pressure area or back to an inlet. A drain passage allows leaking oil to return to an inlet or a reservoir. Some slippage is designed into pumps for lubrication purposes. Slippage will increase with pressure and as a pump begins to wear. Oil flow through a given orifice size depends on the pressure drip. An internal leakage path is the same as an orifice. Therefore, if pressure increases, more flow will occur through a leakage path and less from an outlet port. Any increase in slippage is a loss of efficiency.
Rear main engine seal or front transmission seals
You need to check the transmission gasket between the transmission pan and the body. If it's leaking you have to replace it.
why do toyota sienna transmission leaking after change a new transmission and ran 5000 miles?
fluid is a lubricant of transmission
If a transmission is leaking fluid, it is likely that a seal or gasket has failed. Replace the gaskets and seals and the leaking should stop.
Work needs to be performed. Also depends where it is leaking from. If pan is leaking just drain transmission fluid and install new pan gasket and refill transmission with fluid. If front seal leaking then you will have to pull transmission and replace front seal.