The rebel is pretty dark, so midnight wouldn't be a bad color match. It is probably going to grow a bit slower than the fescue though. With such a low percentage, it probably wouldn't be noticable though. As for percentage, that is a shot in the dark. When overseeding a relatively healthy lawn, only a fraction of the seeds germinate. If you overseed at the recommender rates, you probably only grow 10-20% new turf.

The only think is KBG growing on a sandy soil will need a lot of fertilizer. I have mostly KBG and I have to fertilize it often in order to make it look good. I highly recommend organic fertilizer as it breaks down slower providing slow constant feeding. Synthetic ferts just leach through sandy soil too quickly but I still use them for extra boost in the spring and fall. I like KBG even though it requires quite a bit of work and yes it will repair itself with time. As a matter of fact I have some landing strips on my lawn from slip and slides that should fill in this fall.

Yes, SSS is very pricey, especially for small batches. Instead of buying your TF and KBG blended, I suggest you get them seperately. Seed them seperately too, you will get more even coverage since you will be able to change the spreader setting to deal with the different size seeds. Stick with the lowest seeding rate as well, this will prevent crowding and areas that are too thick. Don't worry, they will fill in. Also do some internet searching for seed sellers that will sell small amounts. I bought Firecracker last year, had to buy 50 lbs, but it was 90 bucks with shipping. I know the Hogan Company sells small amounts, they carry Midnight II. Pawnee Buttes carries Midnight, $3.40 a pound, but that's the bulk rate, probably be 4-5 a pound for small amounts. If you can't find what you want call the grower, they will give you names of distributors.

For the area where the KBG is not thriving, could that be due to shade? Is it near the north side of anything (tree, fence, building, shrub)? It sounds like you are coming into lawn care with the same knowledge base that I had back in 2002 before getting involved in the lawn care forums. Maybe it's a California thing (I grew up in Riverside and lived all over later on. We even had land in Apple Valley). With your current understanding, you're going to have to unlearn some things before your lawn gets better. For the future, never till your lawn and never add topsoil unless you need to fill a low spots. Your experience is exactly what you get - no improvement on any aspect of the lawn. You will find that your soil becomes bumpy and gets bumpier for three years following rototilling. I don't usually suggest getting a soil test for most situations, but your location concerns me. The best, and possibly the only, soil test that will give you the numbers you need is the one from Logan Labs ($25) in Ohio. The micronutrient of concern is boron, but there are other salts that may need to be tuned up before you can get a nice lawn. Too much boron and your soil will be incompatible with growing grass. There's not much you can do about it when that happens. But it would be good to know so you don't spend a lot of money on a lost cause. Just for grins, how far do you live from Boron, CA?