The black and white squares are in the older Unicode "Miscellaneous Symbols and Arrrows" block so I guess they're allowed. Several things like that are sort of "retroactively" emoji... there's a "display as emoji" or "display as text" character you can put after them.

What HN does and doesn't allow seems somewhat arbitrary, things like the star emoji are in that same block and yet are not allowed as far as I can tell.

Maybe because ■ is part if ASCII?

Edit: ⬛⬜ work, the colored ones don't.

⬛ and ■ are different characters. But it looks like HN still strips most emoji.

It is definitely not part of ASCII.