Nope this isn't bl. Because the official clamp wiki has kohaku as genderless so whatever the prounouns the translator use it's their interpretation. The official clamp wiki has kohaku as a she so that's not right too.
Also just because someone dresses as a male does not make them one. The angels are genderless so pls don't refer to this as a bl it's disrespectful to clamp who actually gave us non binary characters.

The scanlations available before Tokyopop made this a BL series. The angels are technically genderless, but 1) They dress like men when pretending to be human. 2) Hisui & Kokuyo are living as a gay male couple in the sequel "Legal Drug."