Nope, they don't go black, they just become hard.
Haki doesn't turn anything black because it's not literally black.
Just people who assume that because Haki is sometimes represented as Black, that it's literal.
Again, it requires ignoring things like Rayleigh's palms not being black pre-skip, but being black post-skip.
Or, the fact there's not one drawing of any characters during Marineford with black weapons or black shaded limbs.
Just ignore how it's stated that Haki is invisible just to convince yourself otherwise.