Flashing during the day is fine; not blinding. Flashing at night, definitely no, especially the high lumen ones.

I just got one of these:

https://www.amazon.ca/Cygolite-400-L.../dp/B00LXTOT6I

It has a daytime flash, which on the box says NOT to use at night, but especially design for daytime riding. The night time flashing mode is just a quick three-flash while the stead one remains on.