Photoshop has an Edit function called 'Fade', which is basically a linear blend 
from the current of state of the image to the undo buffer. 

This can be very helpful function. For example, if you're using filled selections to emulate 
transparent washes or oil paint glazing, fade allows you to visually dial in the best 
opacity rather than doing multiple trial and error experiments. It saves a lot of time.

Reproducible: Always