It's an anime and a manga....but I've only seen the anime, so I don't know much about the manga.
I'm not in the mood to go over the history, so here's the wiki page for the actual D'Eon de Beaumont. --->
clicky clickyIn the anime D'Eon is a member of the secret police in Paris, right before the French Revolution. His sister Lia was recently found dead, floating down the Seine in a coffin. D'Eon investigates the strange circumstances of her death and uncovers a bizarre world of taboo and mystery. On top of that, Lia's spirit sometimes takes possession of D'Eon's body to get her revenge against her killer.
I highly recommend it. There isn't any BL (canon wise anyway) but there is plenty of cross dressing and fun times.
Pictuuuures:
The Four Musketeers - from the left, Durand, D'Eon, Telliagory and RobinD'Eon and DurandIn which D'Eon very conveniently collapses in Durnads arms....again.DurandLia in D'Eon's bodyLia againRobin in drag biggrin D'Eon in drag biggrin MaximillenTrailer....excuse the series pimping, more people need to get into this.
biggrin