That is fine to request/offer under M; the M rating encompasses any and all of these options: "Fade to black" sex, D/s relationships/dynamics that are sexual or nonsexual, but if sexual the sex itself is either fade to black or only very briefly described.

So if you only want fade to black sex, you can specify that via DNWs and/or optional details but still request the M rating. If someone is requesting the M rating, they are willing to receive any of the above options, so if you only want to write fade to black for M, that is also fine.