First, both are great and I'm sure you will be pleased whichever way you decide to go. Obviously the adjustable sights on the Blackout allow you to realize the versatility of a .357 revolver which will shoot everything from light loaded target .38 Specials, through the midrange and Plus P .38s, all the way up to full power .357 Magnum ammo.
If you go with the Blackhawk, there are actually a couple of choices. Until, say, 2005, Ruger built all of the Blackhawks using the same cylinder size as the Super Blackhawk .44 Magnum. The blued .357 Blackhawk came with that big honkin cylinder, and an aluminum grip frame and ejector housing (the stainless version was of course all steel. Those guns seem to me ill-proportioned and I don't care for the aluminum parts. The large frame Blackhawks also have a quirk in that the chambers don't match up exactly to the loading gate which makes them a little "fiddly" to load and unload. Ruger made the fixed sight Vaquero (what some now call the "Original Vaquero") on that same frame.
In late 2004 Ruger announced the "New Vaquero" which had a smaller cylinder, making the gun roughly the same size as a Colt SAA. The New Vaquero also features a "reverse indexing pawl" which makes the chambers line up perfectly with the loading gate. Here is a link to an article describing the New Vaquero:
http://www.gunblast.com/Ruger-NewVaquero.htm
In 2005 Ruger made a Blackhawk on a New Vaquero cylinder frame as the "50th Anniversary .357 Blackhawk" and because it was about the same size as the original "flattop" Blackhawk from the 50s, Ruger made it in the old "flattop" frame and, like the originals, it is an all steel gun. All of the "Anniversary Model" .357s are blued with a 4.75" barrel. Ruger made plenty of these guns and I see them for sale from time to time in the $450 range. If that finish and barrel length suits you, it is a great revolver.
Later on Ruger made (and makes) Flattop Blackhawks on the so-called "medium frame" of the New Vaquero in .45 Colt and .44 Special. I have a Bisley variant .44 Special Blackhawk and it's a great gun. I always though that Ruger should bring back the .357 in the flattop.
Lipsey's got Ruger to make the .357 Flattop Blackhawk in a 9mm convertible version. Pretty much like the Anniversary .357 but with a choice of blued or stainless, and the choice of 4.75 or 5.5 inch barrels. Here's a link to Lipsey's page showing these and other Ruger revolvers.
http://www.lipseys.com/itemfinder.a...+Series&model=Blackhawk+Flattop&type=Revolver I have seen these in shops for sale new and they were in the mid to high $600s. Very nice revolvers, though.
I started shooting cowboy action in early 2010. I passed up a deal on a pair of Anniversary Blackhawks because I thought the fixed sight New Vaqueros looked more "cowboy". The Blackhawks are good guns for this game and they are legal in the "age based" categories which is what most people shoot. Fixed sight guns are legal in more categories, though, so the New Vaqueros turned out to be a good choice for me since I ended up gravitating to "Gunfighter" and black powder categories later on. That may not be relevant to you but it may be for someone looking at the choice between the fixed or adjustable sight Rugers.
Good luck in your decision making and have fun.