I believe it is the other way around. Not rav Moshe was a daas yochid in this matter but rav blumenkrantz was the daas yochid in interpreting what rav moshe wrote.
How do you interpret R’ Moshe’s psak to not allow colored powder?
It is also strange to say that Rav Blumenkrantz did not mean for everyone,
where is the evidence of that?
I can’t prove it to you; all I can say is, speak to the family, as I have. I wasn’t told to keep this a secret.