I've seen this one!
I'm an arcade tech that has been working on CRT monitors for about twenty years now.
Your problem is a small capacitor. Usually 3.3 or 4.7 uf 50v.
Capacitors can fail, (do fail) fresh from the manufacturer.
Randy Fromm's monitor flowcharts say "Is the picture distorted or curved" --> "Electrolytic Capacitor failure, this is the most common failure of all. It is best to replace all the Electrolytic Capacitors at this time"
He is right. Replacing all the Electrolytic Capacitors will solve this problem.
Interestingly, the black 'blob' with edges that fade away from the 'spot' will look the same without a signal applied. Turn up the 'screen' knob on the flyback until the entire screen looks grey with retrace lines and the 'blob' will be dark black.
It's a capacitor.