I would say it definitely is (I did, infact 😉 )
Tried without first, and it took ages to work the paint into the blocks and used loads of paint. Even after 2 coats it hadn’t covered all the pores, etc. in the blocks.
It goes on much easier and faster than masonry paint – you can just slop it on an smooth it out(and is much, much cheaper). Painting over it afterwards is also easier and faster than bare block, uses at least 50% less paint and has a neater finish. One coat of paint is better than two on bare block, and two coats of paint is spot-on.
Stick something on the floor when you do it, otherwise it shouldn’t be too messy.