How to disable brush blend mode shortcuts

It’s one of the hardcoded shortcuts which cannot be changed in Photoshop natively.

The only solution would be to override it with other software that binds the said shortcut to a different action, so when you press it the blending mode change doesn’t happen, but something harmless runs instead.

Software options vary by system, but on Windows there’s AutoHotkey.

For example - you need to write a script that will activate when you press Shift and + or Shift and _, which will do a sequence of both commands, selecting the next blend mode, and immediately returning to the previous one, thus not making any actual change to it.

The script for this looks like:

+$+:: {
    Send '+{+}'
    Send '+{-}'
}

+$_:: {
    Send '+{-}'
    Send '+{+}'
}
return

I’ve tested it and it works. 

A potential issue is that when you’re typing and you want to actually type + or _, it will do the predefined action. In the example, it will type _+ or +_, which is not a bad tradeoff :)