You might want to consider having the link placed elsewhere. Linking from Blog A and Blog B in the same niche seems only natural because it's highly relevant. Sitewide outlinks however are not great. If it was my site, I would rather have the owner of Blog B mention me in one of his blogs. A page called "link partners" or something like that is also not great.

Overall, I would say more relevance = higher value of the link.