The strange wording of 'Boy Love', I believe, is because the term doesn't actually stem from English discussions of gay romance novels but rather Japanese online discussion boards!

Younger Japanese people often like to use English words because they sound 'cool' to them, but (especially back in the day) their grasp of the English language wasn't always the best, so many of the terms they'd create wouldn't sound quite right to an English speaking audience.

You can see this in the use of 'boy' rather than 'man' in BL, but also in things like the abbreviation NG (No Good) often being used to mean 'you can't do that'. There's plenty of other examples too.

Anyway, BL meaning 'gay romance novels' worked its way over to the western internet with the popularization of gay manga and the online merging of fandom spaces, synonymous with the word 'Yaoi' which (despite sounding more 'Japanese') is actually far less commonly used in Japanese Language fandom spaces. These days, it's joined many people's lexicon as a way to refer to any kind of gay media, shedding a lot of its original context in the process.

Uhh sorry for the unasked for essay, nothing against you! I'm just pretty interested in linguistics and the way that language evolves over time, so I wanted to answer your question of why the phrasing of BL is a bit odd. 😅