It depends in part on the child. I have one child who frets enormously beforehand -- whining endlessly -- then when the poke is over, it's over for him and all is well. I have figured out that all that whining is way of dealing with the stress of anticipation. Then I have another child who is very stoic, stiff-upper-lip, real trooper going in. And practically faints when they finally stick him and falls apart like a blubbering baby afterwards. It doesn't help that he didn't listen to me about making sure he ate and drank enough beforehand!
So if you have some idea that he can't stand suspense, I might tell him kind of late in the game about the blood draw and only mention that "we are going to the doctor" -- ie, don't lie to him but don't give too many details. But if you have some idea that he is the type of personality that will rail endlessly about being LIED to, mislead, can't trust you...etc....I would be as upfront and honest as possible.
Last, I will suggest that you work on getting yourself calmed down and talking with your own friends (or whatever you need to do) so you aren't a nervous wreck. Kids pick up on that and, generally speaking, if mom is upset, they conclude it's A Big Deal but if mom is calm they conclude it's not really a big deal, even if they find it upsetting. (What I call "keeping it a kid-sized problem".) I have found that how well I am coping with my own emotional reaction makes a very big difference in how my kids face things.
Good luck with this.