I have a 2008 Legend (fire) and I believe it sports Cherry black switches - I have always found them to be tougher than I really liked personally. Removing the key cap shows it to be black underneath (I've seen photos of other keyboards where these are blue or brown); would the Cherry brown be the same as this but with a lighter touch?

My benchmark is either the original IBM PC-XT keyboard (light action buckling spring) or the original Mac keyboard (circa '84-85) - or even the rather rare Wyse-50 terminals that we had (though these varied around the world, the ones I was using were like a very light version of the Cherry switches and I could attain disgracefully fast speeds on those things).

You are correct - your board has the Cherry black switches.

The brown switches have a tactile feedback whereas the black switches don't. Also the activation force for the brown switches are less than the black switches.