Not quite like there is on lj, I found some tumblrs by just googling whatever I was looking for +tumblr, but you can also "track tags" on tumblr, so if you type "Merlin" into the search box, it will show you all the posts tagged as merlin and then you click "track tag" and then it'll put a little link to all the merlin posts on your dashboard. So that's how I find a lot of merlin/colin/bradley stuff.

These are some other merlin-related blogs I likee:
fuckyeahcolinmorgan (http://fuckyeahcolinmorgan.tumblr.com/)
colin-bradley (http://colin-bradley.tumblr.com/)
fy-katiemcgrath (http://fy-katiemcgrath.tumblr.com/)

Also, here (http://ourspacesongs.tumblr.com/tagged/follow%20friday) I have a bunch of personal merlin-related blogs on my follow fridays, so you can check some of them out if you want.

Hope that helps!