I'm doing a little detailing on my car this year and my question regards the corosion undersealing
If the underseal was applied at the factory would it be body color?
My underseal is black but the car is blue.
Could this mean the underseal was applied later?
I'm unsure what to do

under coating was NOT applied at the factory. It was either applied at a company where the vehicles came of the rail road cars to be cleaned and preped for distribution to the dealers or at the dealerships them selfs.. it was black..

The undercoating was applied after the cars were painted. So long as the remaining undercoating is still adhearing to the car you can apply more right over top of it although it will make the old undercoating look really dull. If it's a pretty visable spot you'll want to apply it over the entire area so it looks even.

RestoRick sells the undercoating that resembles the factory texture, but you do need a special gun to spray it.

Thank you gentlemen you've been most helpful

From what I've observed, the stuff in the trunk that looks like undercoating (I guess it was to protect the quarters from dings) was applied before the quarters were welded to the car, therefore, they were painted. The wheelwell undercoating was applied after paint, but before suspension, so it is always black. The general undercoating is applied after assembly, I think, because I see it on things like the parking brake and gas tank. Someone said it was not applied by the factory? Is it done as a factory option, but done by a vendor like the Hurst cars and Daytonas, or is it a dealer option?

All cars received wheelwell undercoating that was applied after paint.
As mentioned sound deadening on the inside of quarter panels (& doors), was applied before assembly on the car and was painted over.

Cars ordered with the J55 sound deadening package received additional undercoat on the floor pans and an underhood pad (on non fresh-air hood equipped cars).

All of the above were applied at the factory.

Dealership & aftermarket applied undercoating was usually a thinner & smoother appearing material.

Rick