Can I put black paint on a cow mangler?
No, you cannot. It is only possible to use paint on Cosmetic Items.
At one point long ago, you would have been able, however, to paint a Cow Mangler. Though this only worked with team-coloured paint cans.
This is no longer possible, and these painted weapons are incredibly rare to find.
You cannot put paint on a weapon.
The Paint Can is a tool that allows players to change the coloring of most Cosmetic items.
-TF2 wiki, emphasis mine