It seems a bit weird to me, but there are other ways a blog can make money. Is it tied in with an on-line store? Does the blog owner ask for donations? I would guess that it isn't the only on-line venture that this person has and maybe they use it to drive traffic to another blog or site. I wouldn't worry about it too much. Give it a go and hope for the best. If they don't follow through on paying you, you can just back out.