A bulb can turn black due to a few reasons, primarily related to overheating or a failure in the filament. When a bulb is subjected to excessive heat, the materials inside can degrade, leading to blackening from carbon deposits. Additionally, if a bulb burns out, the filament may break and burn, causing the inner surface to darken. In some cases, a poor electrical connection can also lead to overheating and discoloration.
A light bulb turns black when the filament inside it gets too hot and starts to burn, creating a layer of soot on the inside of the bulb.
There is a black o-ring that you turn to release the bulb, once you turn it you may have to jiggle the bulb a little to pull it out. I find it easier to unplug it first then turn the o-ring.
the front bulb needs to be replaced.
Pull away the black plastic cladding in the back rear corner of the trunk that covers the rear light housing, the turn indicator bulb is the bulb closet to the side of the car, turn and pull the socket and bulb out of the housing, turn and remove the bulb to remove from socket.
Rusting
It could be the bulb that is burned out, but more than likely it is the socket the bulb connects to.
Mold
Check to see if the brake lamp and turn signal lamp are the same bulb. If so, its a bad bulb. See sources and related links below to lookup bulb information.
Incorrect size bulb installed.
Gravitation.
An open ground or a short to ground when the signal is on. Usually a bad bulb.
Brake dust!