I haven't come across blank cards myself, but if I did I'd probably make the most of them, maybe drawing a symbol or image on each that has personal meaning and folding them into my readings with my own definitions.
My mini Rider-Waite actually came with two "art" cards that aren't part of the Arcana, but I use them anyway because they tend to pop up in ways that make sense to me. One shows a couple with seven doves above them, which I read as pure love, kind of like the Two of Cups. The other shows a grandmother in a rocking chair surrounded by grandchildren, and I take that as unconditional love, a mix of The Empress and the 10 of Cups. I honestly enjoy adding those extra layers; it makes readings feel more personal.