In my opinion, the Blackout is really only worthwhile if you can suppress it. Otherwise, it's just a slow and weak .30 caliber.
In a small frame AR, the 6.5 grendel is tough to beat for almost any application, from long-range target shooting, to hunting, etc. Mild recoil, excellent ballistics with all of the great 6.5 bullets out there and almost total interchangeability with standard AR parts, aside from mags, barrel and bolt. If you reload, the 6mm Turbo 40 Improved(6.5 Grendel necked down to 6mm and fire-formed to blow the shoulder to 40 degree) performs even better, but is a bit more work. I've shot about 3500 rounds of 6T40 in the last year and I am completely sold on it's capabilities. From 600 yards and in it's as good as anything and better than most and it holds it's own out to 1k if the wind isn't too bad. It is my go-to round for NRA Highpower competition.
If you're looking for a large frame, .308's are plentiful and can be had pretty reasonably. I've got a DPMS LR308 with a 24" upper that is a 3/4 MOA shooter and a 16" AP4 upper that will shoot about 1.5 MOA out to 600 yards with a 6x scope shooting off a backpack. IF you are looking for more performance out of the 308 sized platform, rifles can be had in .243 and .260, as well as other calibers.
If I were you, Id just buy this, from me:
viewtopic.php?f=49&t=25258Though I admit, I'm a little biased....
Good luck!