I believe Sigma makes these filters themselves. The blue coating is unlike any other manufacturer's multicoating that I have seen before, and furthermore, is found on the Sigma EX-series lenses.
I had one and never had any issues with it, pictures looked great. I would still have it if I hadn't dropped it into a river in the Smokies one day last summer. That being said I went with a B+W the second time around, no problems with that one either, I used both on the 17-40 and 28-70.