I just use a chunk of scrap wood to hold the blade. I don't tighten it very tight - just snug it up, but haven't had any problem at all with the nut coming loose.
I have a big boxend wrench that I found someplace that just happens to fit the nut and I keep the wrench >For years I used a pair of Vise Grips to hold the blade while I took the arbor nut