Use black adhesive tape (or even "metalized"), that works for all hardware that I tried it on (including my RB2011).
Dos it work for the blue LED of RB4011 as well? Or does that one burn through adhesive tape? LOL
Well the normal black adhesive tape shows a dark blue spot when taped over the typical MikroTik powerled (which shines a blue spot on the opposite wall).
Good enough to prevent eye damage and still be able to see that it is on.
However I still have some "write protect stickers" from the old 5.25" floppy days which are made from metal foil, and the LED has no chance against that.
(I think that also was the reason for those stickers, as some drives would use optical sensors to see if the disk is write protected and black tape would not be sufficient)