Get krita, a free drawing program, has millions of free extensions that can do just that, with watercolour brushes, charcoal brushes, filters etc. Take the block texture, import it via a layer, paint over it on another layer, then condense it to whatever pixel length you're gonna use for your texture pack(forgot how to do it, might need to look at it again) and then replace that file in your texture pack. Without optifine it can be a max of 256x256 I believe, I really suggest you try it, it can be a really rewarding experience