Acrylic really doesn't block, per se. You could flatten out the edges with a little steam to make them easier to sew, but the actual sewing will flatten them out, too. If they're too difficult to work with, you can either carefully steam them a bit--don't touch them with the hot iron, or put a damp cloth on them and let it dry. You can pin the pieces down first.
"Trust the pattern!"
Time flies like an arrow, but fruit flies like a banana.
Hop aboard while we follow the cloud-covered van of knitting harmony, man!! Woooo!! --Hedgehog
IngridKH on Ravelry