Thank you Travis Simpkins for your info about these. It's so interesting I had to share:
"From the photos, they certainly look like blank calling cards. Calling cards were used during the entirety of the 19th Century. Your cards appear to have a bit of acid in the paper, so they'd be from the latter part... 1880's - 1890's. The ladies' attire would fit into that period as well. They might've been part of a "season appropriate" set, with these ones showing Winter. Are the others all the same? The colors still maintain their intensity because they were most likely printed by way of chromolithography, a time consuming method in which each color is printed separately and allowed to dry before moving on to the next color. The resulting images are very vivid."
If these were indeed printed via chromolithography, it is pretty amazing because the images are so tiny - about 1" squared - seems like it would require a pretty meticulous hand! The paper is getting some brown spots on it - mold perhaps? I'm wondering what is the best way to preserve them.