backhead

As nouns the difference between blackhead and backhead

is that blackhead is (medicine) a comedo, a skin blemish, a type of acne vulgaris, where a pore becomes clogged with a dark, hard, cheesy keratin-filled substance forming a hard black "head" on the skin's surface while backhead is the wall of a steam locomotive's firebox, located in the cab, where the controls are mounted.

As a verb backread

is (internet|slang|especially in irc) to catch up on an ongoing conversation, by reading previous portions one was not present for.

As a noun backhead is

the wall of a steam locomotive's firebox, located in the cab, where the controls are mounted.