So, I've been playing with my Mitsubishi 65-413 CRT. I've tried to adjust the focus, but no matter what I do, the Blue still has a blur to it. Red looks beautiful. Green comes out very good, but the blue just won't create fine lines. I don't know exactly what blooming looks like, but from descriptions I've read, that's what it looks closest too.

Now my set is 4 years old, and while I didn't know all the stuff I know now when I got the set, however I always kept my Contrast around 50% so I'm fairly certain that it's not blooming because of High Contrast. I get the blooming effect even when I turn the Contrast way down. It doesn't go away until the Contrast goes to near 5%, but by then the set is hardly visible anyways so I can't tell if it actually goes away or it's just not bright enough to display properly.

What could be causing this problem only on Blue? Is it fixable?

Focus is always poorer on blue on these sets, even after careful mechanical and electrostatic focus adjustment.
You don't want the blue to bloom, but I believe that you're not really seeing blooming if it's still there with the contrast nearly all the way down. One thing to keep in mind with these Mitsubishis is that up to 50% contrast the contrast control behaves normally. From 50% to 100% it only changes one click for every 5 clicks you move the control. I believe 31 is midpoint or 50%. So going from 26 to 31 is 5 clicks worth, but going from 31 to 62 is only about 6 clicks worth. Having the contrast at 50% is therefore only 6 clicks or so away from having it maxed out.

Your eyes are causing the problem. It is very difficult to focus well on blue, it will always appear blurrier than the others. The best way to find the tightest beam focus is actually to observe the point during the relatively large range where blue appears as focused as possible where light output drops a bit.