I absolutely love the look of clawfoot bathtubs with their undersides painted black. Matte, glossy, semi. Whatever. A high end fixture manufacturer (I won't name names) has been making this tub for awhile now and it. is. pricey. Absurdly pricey. On a recent restoration project I was working on, we proposed using aforementioned tub, which the client loved, but budget restrictions forced us to look for an alternative.
We ended up using a less expensive but lovely clawfoot tub with a white underside and had the painter paint it and the legs black. It cost us one third what the other tub would have. He used an industrial spray-on enamel, something you can find at a good paint store like Sherwin Williams or Benjamin Moore. I also think it is something you could definitely DIY, with someone to help with the heavy lifting!