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Sequence observed.

Good Pilot, call for heat, valve opens, good flame*, no blower, valve closes.
Fan switched to ON Blower works.

My first thought is a limit switch, can they be Jumpered to see which one?
Any other thoughts appreciated.

If it is a multispeed blower you could have a fried/bad winding on the heat speed. You could also have a bad fan switch. I don't see too many limit switches go bad so I would start at the fan switch.