are any apps limited to using commercial limo services?

and speaking of, seems at least some limo services are getting into copying uberX etc.

Uber Black uses commercial limo services.

that has seemed unclear to me looking at uber and websites targeting uber drivers

there are also other limo services that (now?) seem to accept anyone with smartphone

Correct, at least in the US (and I believe EU as well), to operate an Uber Black you have to have a commercial D/L and the car has to be registered under a commercial registration (or livery as applicable by state). This of course also impacts insurance coverage as commercial insurance requirements are much higher than standard drivers. Uber Select is for higher end cars that average drivers (non-commercial) have.

Pretty much Uber Black is going to be local companies that supplement out their normal reservation process with Uber Black (though its possible to be completely independent as well). In NYC is very common to see cars registered (tags in the window) for a general black-car car service that you would book through a major company ahead of time operating as Uber Black.