Blocklayer price usually includes, blocks, fitting lentils, cills, insulation, etc, up to wall plate but only the labour. You buy all the materials.
Personally, I would let the blocklayer buy the material, use it as he needs it and pay him when job complete. This way, they're getting the blocks they want, etc and will probably buy all of it cheaper than you anyway. Also less chance of theft as they'll get what they need for the week or whatever, plus the cement will be fresh. If they offer to buy the material rather than yourself, don't question the offer and stay friendly with them because 99.9% want to do a good job and take pride in that.