So--is it safe to say that if a yarn is machine washable, you can't really block it?
I'm not talking just about synthetics like acrylic, but also superwash yarns, like superwash wool. Does the process of making it superwash also make it unblockable?
You can try steaming and pulling it into place, but a lot depends on why you're blocking it. The main reason is to even out the stitches and for both acrylics and superwash, washing and drying will do that.
I usually block sweater pieces more to make them easier to seam in addition to evening out the stitches.
I'm thinking about knitting a sweater in Elann's Superwash Bamboo, which is 65% Superwash wool and 35% bamboo...maybe the bamboo content would make it more blockable.
I'm not so sure that bamboo is all that blockable in the first place. Go ahead and try it, wouldn't hurt, might help you in your seaming.
I've tried blocking 100% bamboo and not had any luck. It doesn't have any memory and is very limp. Made a very soft scarf though.
You should be able to wet block it - just swish it in water, roll it in a towel to squeeze the water out, and then pin it to the correct measurements.