I may have fucked it up on 4bpp devices ^^.
EDIT: Yep, that's a 4bpp-specific issue. I'll see what I can do

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EDIT²: It's still honoring fg/bg pen color for anything not glyphs, that's what the not terribly clear mention in the help message meant. i.e., fbink -F block -o -S 4 ABCDEFG -B gray2 does what you wanted

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EDIT³: That sounds sensible, but it's a tiny bit annoying to actually make happen ;p. (And it'd have to be a different flag than -c). (FWIW, for a blank line/page, ' ' (i.e., a single space) works, while '' will get you shouted at

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