Hi IronTom,
Welcome to PA. I started by reading links on this site and my first book was the Traditional Bowyer's Bible Vol. 1. There are many good books, I'm sure other's will chime in with their preferences.
Not sure which species you mean by blackjack. Black Oak is about a .6 specific gravity.
As far as "scrub oak" I haven't used it yet, but am starting to eye a few of them myself. I've got 'em growing around me everywhere.
I think it's supposed to make fine bow-wood. Wondering myself, is scrub oak actually post or swamp white oak? I call it scrub oak myself but am thinking it is really named swamp white oak? The Bowyer's Bible Vol.4 mentioned it to have a higher specific gravity, I think around .7, only surpassed by Live Oak at .85.
I think I'm going to cut some soon. I've found a couple of interesting trees.
The trees you are mentioning at 7" diameter would be plenty from what I understand. Considering for a 67" long flatbow made from such a stave the width of the limb coming out of the handle fade would probably be around 1.75" or less, maybe.
I live in SW Florida so the scrub oaks grow year round, the wood doesn't have growth rings. I'm planning on just removing the bark and going from there. When I cut some, my plan is to leave the bark on and seal the ends with old glue, paint, or wax, whatever's handy. Then just let it sit for a long time, the rule of thumb is a year per inch in each direction of thickness. This can be sped up by reducing the stave to near floor tiller dimensions, then left to season for several weeks, and their are some other techniques people will use, including hot boxes, or smoking. But speeding up the process can with some woods lead to drying checks, twisting, or other issues. I'm not sure how the oak will fair.
You may want to consider starting with buying a hickory board with pretty straight growth ring lines to start with. It's a great beginning place.
I'm warning you though, I had no idea how addictive this would all be when I first threw my hat into the ring!