Maho is blood magic in Rokuganese, period. It was once safe, but Fu Leng's fall opening the Festering Pit and releasing lots of kansen changed that and made it very risky, so it was forbidden by Imperial edict.
Using blood magic to call upon a kami doesn't cause Taint. Doing the same with a kansen, does. Problem is, sometimes you don't know what you are getting; kansen are tricky, they want you to fall. Somebody experienced like Tadaka probably can tell the difference... 99% of times, under controlled circunstances and in the relatively safe Isawa lands. Now, he means to go to the Kuni lands. This is not going to end well.
Edited by Mon no Oni