I used to work with every colored element being on it's separate layer, but when you have a complex painting, the amount of layers makes it very dificult to navigate, clutters the whole project file and also makes the file way bigger than it should be, thus slowing the process of painting and decreasing performance because of the increased size of the file. That's why I switched my method to selecting stuff on one layer but then I ran into this problem which I've described. This seems like a bug, I don't see how it could be a feature because when I select something, I want nothing from outside the selection being dragged inside, the way it is right now partially defeats the purpose of using the select tool if you use the blending approach. It would be preferable to at least have an option to toggle between sampling from the whole layer and sampling only inside the selection.
I must say that I am in love with this app, it's the best painting app that I have ever used. Please consider my suggestion, it would make the experience of painting in Artstudio Pro even better for people that use blending brushes. Thank you for working hard on improving the app, I really do appreciate what you're doing.