All post filters get carbon dust on them. The carbon brushes are always wearing down in use, and all that has to go somewhere
It's why post filters never come clean when washed, the carbon stains them. You can wash them clean (pads you can anyway if their not too ruined) but they will always be stained black. If the motor is good it will take a long while, I've had dysons back a year down the line for something that I've put a new motor in, and the post filter is usually just starting to go a bit grubby, but not black.