Could need to be degaussed, especially if you recently repositioned it. Most if not all somewhat modern CRTs degauss on power up (the boing sound you hear on power up) so it's normally cured after several on/off cycles (ALWAYS let it cool ~20 minutes between cycles or it can fry the degauss circuit). Some CRTs have a manual degauss in the menu that can be performed while the TV is on (SAME cooling rule applies with this method). If this doesn't cure it see if the 'blob' changes when you shift the position of the TV 90 degress right or left (i.e. it's an earth magnetic pole thing).
I have an old CRT that the degauss circuit fried on so I had to manually degauss it from the circuit of another old CRT. It's dangerous so I'm not going to suggest it here unless you have electronic experience.
Other than that the picture tube is just getting old, maybe a circuit issue, or one of the magnets on the back of the picture tube fell off (glued on and can get dislodged with banging around).