The big question is, what are the tooth counts of your chainrings and rear cogs? That BL is a long cage derailleur designed for a lot of chain wrap. The NR can't possibly wrap that much chain unless you buy a Soma Rally-type cage for it. I'm guessing that the prior owner of the bike put the BL on there to compensate for a large freewheel or wide range set of chainrings. You need to get this info straight before proceeding.