I haven’t come across blank cards personally, but if I did, I’d probably make the most of them — maybe draw a symbol or image on each one that has personal significance and then include them in my readings with my own meanings.
For example, my mini Rider-Waite deck actually came with two "art" cards that aren’t part of the Arcana, but I use them anyway because they always pop up in ways that make sense to me. One shows a couple with seven doves above them — I take that as a sign of pure love, kind of like the Two of Cups. The other shows a grandmother in a rocking chair surrounded by her grandchildren, which I interpret as unconditional love, similar to The Empress mixed with the 10 of Cups. I find it really fun to give extra layers of meaning to cards like that.