It depends how thick your beard is. If its patchy then it wont work as it needs hair to grip on to, if its thick it could work. I use dermmatch for my goatee
Yeah, I would think that something like Dermmatch that you kind of "paint" on would be better for a beard than a fiber concealer. Especially since the beard hair is upside down most of the time(unless you live your life standing on your head). ;-)
I use dermatch on my eyebrows and this helps a lot to spruce them up. Although, you need to have the hair there in the first place before doing this otherwise it'll look like you've painted your face.