My grandma gave me these black trivets when I was 11 years old. She thought I should have them because they were from the '50s. It seems strange for a little girl to have her own trivets, but I did.
The trivets were manufactured by Frankoma, although they're not in the usual Frankoma style. They are solid black with a fleur-de-lis design. Frankoma pottery typically comes in earthy colors, like green, gold, and brown. Sometimes the red clay is allowed to show through the raised patterns, like the trivets in the photo below.
Although I don't really collect Frankoma, I have several pieces of it. The Frankoma factory is located in Sapulpa, Oklahoma, which is a town west of Tulsa. I suppose it's fairly common for people in this area to have a few pieces of it, even if they don't collect Frankoma.