How do I remove the blade from my Black & Decker electric lawn mower?

Before anyone complains, yes, I've already looked on google, and what it
says is "Remove the nut that holds the blade"! Duh. (And what is the
point of newsgroups if one has to google things first?)

I have a half-inch socket wrench with a T-bar and I put a 16" pipe on
that. I turned the mower upside down. To stop the blade from turning,
I put a 2x4 between it and the wall of the house. I turned the wrench
and the whole lawn mower moved!!!

I sprayed the nut/shaft with PB Blaster, and then I hit the nut with
medium-weight pipe to vibrate things. Eventually did this 3 or 4 times,
over 4 days.

My version of an impact wrench... I hit the wrench with a short piece of
fence post. Didn't work well.

So after trying various arrangements, I used a 2-foot piece of
semi-round fence rail to stop the blade, put the mower on its side and
held the rail against a rear wheel, and put a 3-foot extensino on the
T-bar, and voila, the nut rotated. Changed to a ratchet handle, and
kept unscrewing. But no progress. ;(

Now the blade stays still but the nut, the washer, and the shaft all
turn together.

I have the kind of impact hammer that you hit with a hammer and it turns
the downward force into rotational force. Wouldn't that damage the
electric motor at the other end of the shaft? (I have used it in the
past and it worked, but never on an electric motor.)

How do I get the blade off?