How can I block acrylic / wool blend yarn?
Data point: I successfully blocked a mixed-fiber scarf by soaking in warm water! It was made of fingering weight yarn, content: 75% superwash merino / 25% nylon.

For an arbitrary mixture, the safest method would definitely be to make a test swatch and see how it responds... but since I don't always swatch, the next best thing is to try blocking in order of least to most potential for damage.


Block by soaking and letting dry
This will only be effective on the natural fibers in the yarn. This will not have any effect on the plastic fibers, but will not damage them either. In my case - 75% natural fiber - this was sufficient to retain the shape.
Steam block
This will affect both types of fibers, so it should be effective. The danger here is that you over-heat and melt the plastic fibers. This is permanent! So I would only use this if the previous step did not work, and on low heat. Wool and other natural fibers won't be damaged by that either.