I'm afraid I don't understand the question. The blend space should generally be set to match the color model of the background.
I'm wanting to know what to set the colours to while I'm working on the document to make sure the colours I choose are going to be the same as what I print. I'd use RGB while creating a document onscreen then change to CMKY when I'm printing to maintain a true representation of colours. Can I do the same thing using the transparency blend space?
It will have to be converted to CMYK for printing. However, it may be converted before or after transparency blending, and it might not be converted by you. You should not alter your colour settings once you start.