Assuming that at the mechanical fixings have been slacked off adequately I'd be looking at it being held by rust or wood resins.
Have you turned this particular blade over previously? If so, maybe the blade had a residue which has compressed and stuck hard?
If you can stop the cutter head turning and hit the edge of the blade hard with a punch you may be able to shatter it....
Heat is a good sometimes for stuck things.
With extreme care, as this isn't an ideal situation, I'd have a fine blowtorch across the length of cutter block and blade. Prepare for rapid application of a punch on the blade, either on end, or as above, on exposed cutting edge and a damp cloth to cool things rapidly.
Be careful with the heat.
Cheers Andy