Blend Modes Hotkeys are broken
Illustrator allows users to set custom hotkeys for blend modes, in Keyboard Shortcuts > Tools, right in the very bottom.
Aside from direct commands for each available blend mode, there are two contextual keys: Next Mode and Previous Mode.
They don’t work as expected. Let’s make a simple test.
1. Assign Shift+9 (displayed as 'Shift+(' — which is not correct, actually) for Previous and Shift+0 ('Shift+(') for Next blend mode.
2. Put two colored shapes in a stack, to see what’s happening when you change modes
3. Open Transparency panel, to see which mode is chosen (for some reason Properties does not allow to see the mode)
4. Select the top shape and hit Shift+0 (Next Mode)
...nothing happens. Illustrator refuses to change a mode when it set to Normal. Shift+9 (Previous Mode) also does nothing.
- Change the mode to Darken manually via the panel
Press Ctrl+0 (Next Mode)
Now it goes correctly to the next mode, Multiply.Hold this Ctrl+0 hotkey
Ai goes through all available modes and stop at Luminosity, the last one — and won’t loop to the start of the listHold the other key, Ctrl+9, to cycle back
Ai never stops looping through all modes, but won’t never display Normal one.
TLDR:
A. Next Blend Mode hotkey does not work when Normal is chosen
B. The list of modes does not loop to the start when you use Next Blend Mode key
C. Looping through blend modes omits Normal blend mode
Why is this important?
A minimal way to utilize these keys would be one to set a normal mode (to reset blending, basically), and then any counted number of presses to navigate to a desired one. Instead we have to assign one for Normal, and one for non-Normal and hit the wall when cycling down.

I am happy to share that we have fixed this bug in our latest release – 27.0.0 which is available worldwide now.
Going forward, our goal is to fix as many top User-Voice bugs as possible and as frequently as possible. Given the nature of the fixes, some of the bugs will take a longer time to fix, but we are on it.
You can update to the latest release using Creative Cloud desktop App: https://helpx.adobe.com/in/creative-cloud/help/creative-cloud-updates.html
Thank you for all the feedback. Keep it coming!
Ankit Goyal
Illustrator Team
-
Photoshop has default hotkeys for looping through blend modes, Shift+Plus and Shift+Minus. Illustrator can have these assigned too (in Keyboard Shortcuts, in Tools section), but they behave differently — when the end of the list is reached you can't jump to the beginning.
That is pressing Shift+Plus does not take you to Normal from Luminosity. Strangely enough, Shift+Minus does choose Luminosity when you are on Normal!
To maintain consistence between apps, this should be changed.