Not necessarily a blackboard. Any black surface would do. As long as it was quite flat and evenly lit, and nice and dark. A wrinkled black blanket would be a bad idea, if you know what I mean.
The way I see it, there are two options:
Start with the photo of the baby on a black background, and Place the picture of the drawing, and set it to "Screen" blend mode to merge it. This way should be the easiest, if we can pull it off.
Start with the photo of the drawing, and Place the baby onto it, and mask it.
Either method should give great results. I can't wait to work with you on this project!