I would define custom work to be real Gunsmith Alterations.
Magnaporting
Slab Side the Barrel
Exotic Sights requiring more than a simple replacement.
Custom Trick Finish
Replaced Hammers, Backstraps, ejector rod shrouds, etc.
And probably a combination of several of the above.

My SS Colt XSE 1911 45acp Pistol has been sent off for checkering of the front strap. It has custom sights. It has had a Wilson Combat Grip Safety installed (Not the Drop In One). The Hammer and Trigger are Wilson Combat.It has a Flat SS Mainspring Housing with Magazine Chute attached also Wilson Combat. It is on the second set of Custom Grips. The first set was Ebony with Double Diamond pattern checkering. The present set are Black/Grey Two Tone with a StarBurst pattern. That is only part of the work that has been done to it over the last 20 years. I would call it Custom.

I have a 625, 610, and two 686 S&W's that are Power Custom Combats. I am not sure that they would really meet my requirements as a Custom Revolver.

Keep on working on that BlackHawk it may qualify as a Custom sooner or later. My BlackHawk came pretty trick. It is a Lipskey SS 5.5" Bisley 45 Colt with factory Rosewood grips. I have had a spring change done to it. Other than possibly some Throat work and possibly a new front sight if I can ever come up with a Fiber Optic for the 45 Colt I do not see much being changed on it. I do not see it ever qualifying to be a Custom Gun.

Just My 2 Cents

Bob R