Jester made that comment a while back and wound up generating a fair bit of discussion across several blogs. There definitely are market blogs out there though there aren't a whole lot and many of them are either inactive or not very active.
This post is intended to highlight the market blogs that are active now.
As far as I'm concerned including my own blog there are 4 main trade / market blogs for Eve Online. There are others but the rest are either too new, too small, inactive or not active enough to be good sources of current information for market players. Most of those other blogs can be found on the sidebar of this blog.
The 4 Main Market Blogs
Merchant Monarchy - Author MoxNix (me)
I tend to write mostly about playing the market and related activities though I do write on other topics occasionally. I'm not as prolific as some, generally producing 8-12 posts a month though there are times I'll crank out a post a day for a week or two and wind up with 20 posts for the month.
Greedy Goblin - Author Gevlon Goblin
Quite prolific (generally produces a post a day, sometimes 2), Gevlon is a long time blogger with a large following of readers from other games. Gevlon writes about a lot of different things but his recognized area of expertise is in playing the market. His market theory is sound and his market related posts are good though much of the rest is controversial to say the least.
MarketsforISK - Author Croda
Croda writes pretty much exclusively about playing the market (so far anyhow) and produces 10-12 articles per month. His is the newest blog of the bunch though he has prior experience blogging about making gold in another game.
Low Sec Lifestyle - Author Sugar Kyle
Far from being just a market blogger, Sugar is a very prolific blogger who writes about many different things, far more than just trading. Though trade is not her main focus, she writes market related articles often enough and the articles are good enough to be considered one of the more important market blogs.