The blogs login system is slightly different. The username must be in all lower case letters. We see this problem a lot if your blog username and your forum username are the same, but your forum username has an upper case letter. The 3FC cookies will likely remember your forum name, attempting to force the upper case in your blog login as well.
However, the forum username will let you log in with all lower case even if you have upper case letters in the username. It will display correctly when you are logged in.
The way around this, that I've personally found, is to clear your 3fc cookies completely. Then log into the blogs first, setting a new cookie using all lower case letters. You can then log into the forum also using all lower case letters.
That should take care of it
Some browsers may have different systems of holding your login info, so you may have to make adjustments.