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Most true focus problems that I have encountered (when the IHVT is ok) are related to leaks or resistance on the focus output. The diming of the screen when the focus pot is adjusted leads me to think in terms of a leaky socket. I'd remove the ground from the CRT socket to the tube dag and see if it sparks. If so there may be a leak in the socket to ground. It could also be leaking to another pin, such as the screen grid. A rhetorical question: What happens to the screen voltage when the focus pot is adjusted?
I have seen sockets that had no arching or other telltale signs, leak through the plastic housing to ground out the focus voltage.
Look closely at the screen. If the blurring is in the form of small circles, then you have an open or hi-resistance focus electrode inside the tube. The circles may vary in visibility with brightness.
If you still haven't found the problem, try to confirm that this is truly a focus problem. Remove the CRT socket and observe the hi-voltage. If it climbs more than about 1k, say all the way up to 25 kV, then you may have a beam current problem rather than a focus problem. In that case re-check all CRT board voltages. WARNING: Removing the CRT socket and powering the set may destroy the CRT on some models. See the section: Warning about disconnecting CRT neck board.
If you have done all of the above and removing the socket makes no change in the high voltage, then try to determine why the high voltage is low.
Watch the screen as the brightness, contrast, or screen control are adjusted. See if you can observe any signs of blooming. When the IHVT doesn't provide enough current to satisfy the demands of the tube for current, the the picture tends to appear to expand like a balloon. i.e., bloom. This can be caused by not enough drive to the IHVT. Carefully monitor the b+ to the horizontal drive stages to see that is is stable and correct.