Oh and I meant to mention Design Milk, I follow them via twitter but they basically post links there to their blog lol [link] It's a lot of architecture and product design mostly, but it spices up amoung all the illustration I follow XD
Tumblr is a blogging platform same as blogger or wordpress, if you step outside the fandom (of which I do not follow any fandom based blogs) it can be just as professional as elsewhere depending on where it's being run. And it depends on your definition of "professional" as in "a big ass multinational company is running this blog" or "a artist who gets paid" "a blog that makes revenue" or what? People like Neil Gaiman and Austin Kleon both professional Authors use Tumblr as a place to publish editorials and thoughts which is part of their professional life? Is that not professional?
Design Milk is a site, I just use thier twitter to follow them. They should have a link on their twitter to their blog.
[link] it is a picture blog about sustainable creativity, it is way more interesting than it sounds. I encourage every artist to read all of it, sometimes the lessons sting a little, but it is amazingly inspirational.