A 4.5" open folder won't have much more than a 2" blade, if that. Finding one that small with a combo edge will be difficult. And to be honest, I'm not certain that a combo edged 2" blade would be particularily useful. Only about 1.5"-1.75" of the blade will be actual cutting edge, and half that in serrations isn't very much. The purpose of serrations is to increase cutting edge when slicing...and there's not much to slice with.
The Kershaw Ken Onion Chive comes in black/black, comes close to your size criteria, but I don't think it's available combo edge. The Scallion comes a bit larger and has models available with combo edge, but I couldn't find any black bladed combo edges. *shrug* Both are pretty dinky regardless. The recurve of these blades makes up a lot for their short length.
Spyderco makes lots of smaller knives, but one thing you won't find on a Spyderco is a thumb stud. They only do a couple models with a black coating, and these are usually larger models.
Oddly, the closest I can find are actually autos. Protech makes several different 2" autos (CA legal) with black, partially serrated blades (you can see a couple
here, but always shop around for the best price--these seem a bit high). The downside of course is the knife might not be legal where you live, and the price will be 2-3 times your $50 limit. If it
is legal where you live, I say bite the bullet and get one. Protech is one of the best automatic knife companies, the ATS-34 or 154CM blade will be far, far better than the 420J or AUS6 you're likely to find on a sub-$50 knife, and the knife will last a lifetime. Plus, it's just freakin' cool.