If you open Keyboard Shortcuts, make sure you have Tools selected, scroll all the way down, you’ll see all 'Blend Mode - [name of the mode]' commands there — empty, ready to be assigned... but. Opt/Alt cant’ be used for these. Because these are in tooools. Which means these can be bound only to naked keys and Shift+keys. Which are all taken, long time ago!

So please move Blend Mode keyboard shortcuts from the Tools section into the Menu Commands section, so we can use all possible modifiers for these, and not only Shift.

Also, why do we even have this limitation? What law forbids me to use, say, Alt+7 for a tool?

So please also vote for these:

— Allow all modifiers to be used for anything anywhere:
https://illustrator.uservoice.com/forums/333657-illustrator-desktop-feature-requests/suggestions/34616848-support-for-more-keyboard-shortcut-modifiers
and also this one:
https://illustrator.uservoice.com/forums/333657-illustrator-desktop-feature-requests/suggestions/43161018-remove-restrictions-to-useable-keyboard-shortcuts

— Allow to map anything to a hotkey, including all the panels’ menus:
https://illustrator.uservoice.com/forums/333657-illustrator-desktop-feature-requests/suggestions/31609237-assignable-keyboard-shortcuts-for-everything