Hmm Yami no Matsuei... It's been some time since I last read or watched it. But it's definitely on my favorites list.
The series is a little confusing with the amount of innuendo and the characters' relationships (not to forget the background stories), it feels like yaoi but there's never anything more than hints and playing with the reader's imagination. Although I guess the most definite hints occur whenever Muraki enters the scene ^^. So I think the answer to your question might be that YnM contains a lot of BL innuendo but does not cross the line to actual yaoi (at least not in the available volumes). There's good fanfiction around for that though.
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The series is a little confusing with the amount of innuendo and the characters' relationships (not to forget the background stories), it feels like yaoi but there's never anything more than hints and playing with the reader's imagination. Although I guess the most definite hints occur whenever Muraki enters the scene ^^. So I think the answer to your question might be that YnM contains a lot of BL innuendo but does not cross the line to actual yaoi (at least not in the available volumes). There's good fanfiction around for that though.