Here's what I'd ask you -- if you saw a comic that looked kinda like the sort of comic you're making, and it was labelled "BL," would you, personally, be more or less likely to click on it? Would it appeal to you? If more, I think there's nothing wrong with adding it -- it's clear that there's something about that kind of story that appeals to you, and there will be plenty of other people that feel the same! ;u; But if you'd find a "BL" label off-putting, then don't add the label -- that's obviously not the kind of feel you're going for and it would be disingenuous to present as such.
If you wouldn't care one way or another, then why bother! I don't think you are OBLIGATED to warn readers anymore than you would for a heterosexual relationship -- a lot of people are starting to move away from the idea that HOMOSEXUAL CONTENT must be kept separate from "normal" romance in our media, especially in independent media like webcomics, so it's less and less reasonable for audiences to expect a warning for that sort of thing anymore.
If you really want to make sure your audience knows the direction you're going in, though, you don't need "WARNING: CONTAINS BOY LOVE" on prominent display; just mention in the description that it's about this character coming to terms with his feelings for another guy.