I'm guessing the transmission crossmember was painted semi-gloss black. Mine has blue paint on it from the factory. I'm wondering why?

how much history do you have with the car?

40 years is a long time for people to do odd things.

This is an original untouched 74 Barracuda.The paint looks a lot like the markings I found on the differential near the top. I'm thinking the blue paint is inspection marks. I found many of these on this project so far.

If that is the case and in consideration of the location, it would be inspection marks verifying torque up of cross member to frame bolts. Underneath that blue paint is either black paint or bare metal depending on how the cross member support was finished back in the day.

Chrysler had different vendors for some parts, if i am correct;
both my 1963 Savoy and 1963 Fury had light grey transmission crossmembers and k-members.