The blue coolant is genuine BMW coolant available only from the dealer. There are lots of stories bouncing around about green and orange being 'bad' coolants, but there are just as many stories of folks using them for years with no issues. If you have blue already, stick with it.
As far as your leak, just keep looking around. The radiator necks are prone to cracking, as are all the plastic pieces in the cooling system. Check all of the hose connections with the engine cold using a mirror and look for a drop of coolant hanging under the fitting. Good luck.
Mike E.
Yorktown VA