I was looking to buy Federal American Eagle Black 223 Rem 55 Grain FMJBT.  Just wondering what advantage boat tail ammo has in an AR-15.

I've heard  Federal American Eagle ammo is good across the calibers....but have not heard anything good or bad about their "Black" line.  What's the difference in regular A.E. and A.E. Black?

Just from my stockage of AE223 it's packaging, nothing more.  Have shot some along with some of the older red-box (and a couple other different package schemes) and performance is consistent in both the rifles it's been fed to. The part code on the box is going to tell you what it is. AE223(then a suffix determined upon box-quantity/packaging). But AE223 is AE223.

I’ve too read somewhere that the part after AE223 describes the packaging.
Anyway I have some Of those AR 223 branded AEin black boxes and they work just fine in all my ARs.

For most purposes with a carbine usage out to a couple hundred yards, very little difference.  Boat tail comes into play nicely at extended distance.  The main advantage of non-boattail is:

-I suppose you get a heavier bullet in ~ the same dimensions (and it'll take sensitive instruments to measure that - most cases doesn't matter)
-It's been said that rough barrels will shoot better if there is longer contact baring surface.  So if you have a really worn and pitted barrel, there's more surface to contact and dampen out things with a non-boat-tail vs Boat-tail.  (maybe).

For most applications, boat-tail is equal or better than non-boat tail, and given the choice, rarely should you choose FMJ over FMJBT (at the same price).  That said, don't be so sure an FMJ isn't an FMJBT, without confirmation.  It's rare for an FMJ 223 bullet to not be an FMJBT, but I suppose some do exist.

Thanks for the input.  I assume there's no downside to using boat tail bullets in an AR-15?  Is the standard USGI 5.56 boat tail or FMJ?

The picture TIG posted is indeed what I was inquiring about.  Someone had a big sale on them around Black Friday.