A friend and I collaborated to make a present for a mutual friend. The recipient of the gift recently got into yoga, and she's a huge fan of Star Trek. So, naturally, we decided to try and decorate a yoga block for her.
It was surprisingly easy, if a bit time consuming due to the complexity of the design. The secret? India ink. It's easy to brush on in thin or thick lines, creates an even coat, and most importantly, it's waterproof! I had originally thought of using a sharpie, and you could probably use one anyway, if that's all you have on hand (especially if your design was simple and mostly lines). But india ink is not too expensive, and lasts a long time. It really made filling in the large black areas a lot quicker and neater than coloring with a marker.
Since our block was a medium purple (and maybe also due to the foam material), the basic go-to of a graphite transfer for the design didn't work. So instead, after the image was on a piece of paper, I held it in place on a side of the block, and used a small sewing needle to prick along the outlines - closer together in more detailed areas, farther apart where the lines are straight. After that was done, it was just the simple, if delicate, process of painting in the lines that were created by the needle. The small holes left by the needle do still show, but only if you look at it very closely.
The block hasn't been used yet, so I don't know if the design would wear off quickly or at all. However, it's a fun way to individualize your yoga block.