According to Catholicism, if we can thank God for our fortunes why can we not blame him for our misfortunes? If God is all knowing and omnipotent and we are to thank him for all of the boons we have received why can we not also "thank" him for all the misfortunes he has set upon us?  

I always see people thank God when something good happens to them, but when something bad happens they never even mention his name.
Actually, it's an apostolic concept to see misfortune as the stuff of sanctification, and thankworthy—"all joy:"


  James 1:2-4 (DRB) My brethren, count it all joy, when you shall fall into divers [trials]; 3 Knowing that the trying of your faith worketh patience. 4 And patience hath a perfect work; that you may be perfect and entire, failing in nothing.


You probably don't see people thanking God for bad things often because, more often than not, people are oblivious to this theology.

In Catholicism moreso perhaps than any other 'denomination' there exists a rich theology of penance, meaning that an understanding is more prevalent that penance is God's gift for sanctification—that He would allow things we are owed for our sins and much more anyway to actually serve to restore us and sanctify us.

The kind of thanks, though, is different between receiving goods and receiving trials, quite obviously; God wants directly to give all people good things, but He wants only indirectly, to solve a problem He'd rather didn't exist, that is, to give them trials.