As far as I know Blitz is optimizing the usage of most things. For example texture, or sounds. If you load them twice, Blitz will load them only once internally, and create a link to the original for the second one. I guess the same goes for Brushes. There may also be some sort of garbage collection, so when a brush was deleted and is no longer used by any other surface, then and only then it will be removed from memory.