It's been years since I worked with that "costume satin" -- when I made a princess gown for my little girl. What I did was wash and dry the satin first, and then I heavily starched it to keep it stable while sewing. The starch helped a lot, plus I knew the dress could be washed later. I would stick to polyester satin, as it should wear pretty well (polyester is tough stuff). I would probably just shop for the thickest polyester satin I could find.
Where did you put the fray block? When I sewed the satin dress I just made extra-wide seams. I think I may have turned under the edges and sewed them to limit fraying, or sewn an extra line of stitching next to the seam. As long as the seams are enclosed, I don't think you need to use Fray Block on them. If you starch the satin before cutting, I think you will find that the starch stops a lot of the fraying while you are handling the satin. Also, if you want, you could cut the seams extra wide (say, an inch) so cutting away frays later won't weaken the seam. And, if you make the seams extra wide, you could trim the seams with pinking shears to deter fraying.