Hi John
As both Don and Gordon have stated, it is difficult to evaluate the wheaten and blue wheatens early. They go through so many changes, especially the cockerels, from hatch to a year old, I've pretty much decided to hang onto whatever doesn't have obvious faults (porcelain in the ear lobes, duck foot, scissor beak) until they are at least 9-10 months old.
The black in the hackles is one thing I look for, but it is one of the last things I look for in cockerels. In pullets, however, if they've got black in their hackles at 3 months, they will have black in their hackles as adults. I will cull for that early on and sell the chicks or started birds as backyard layers.
Other than that, it is very difficult to know how a wheaten or blue wheaten chick is going to color out as an adult. If you have the room, I'd recommend holding on to all the wheatens and blue wheatens you have (you can cross breed those, btw) until at least this fall and re-evaluate then. Feel free to post pictures here for critiques, as well.
You're right about the temperament of the wheatens vs the blacks. I've learned to raise my blacks with my wheatens so they pick up some of their calm, friendly manner. It's worked pretty well, so far - while my young black grow-outs are not as curious (or as quick to jump in my lap) as the wheatens, they are not near as flighty as the ones that weren't raised with wheatens. Wheatens and blue wheatens are two of the more challenging varieties in the Ameraucanas, but that temperament keeps me going with them.