"Double black" Retzuŋaus

Is there any issue with being machmir by using "double black" retzuŋaus on our tefillin according to M"A?

Is this to say that this shouldn't be done, or merely that it was a hakpodoh which wasn't followed? I've heard in the name of the Tzitz Eliezer that it should not be done, but most other poskim have no issue with it, and some viewed it as a worthwhile chumroh, although they didn't require it (the latter being the opinion I've heard in the name of R' Elyashiv זצ"ל).

I heard that maybe there's a health issue with the blank ink being against the skin.