The button of inserting format is visible, but—
When editing a message, the formatting code can be inserted with the button, but will report illegal chat character. Cannot directly type §.
When editing a sign or book, clicking the formatting code only inserts the number or char, but § is not inserted. Cannot directly type §, either.

I tried to put the java files from IMBlocker into the Color-unchained mod (the .mixins.json is merged as well), and try to debug in Intellij, it works fine—formatting codes can be inputted and IM is blocked. But if I undo that, and put the IMBlocker .jar mod into run/mods folder, then things goes wrong as I mentioned.

I found that's because it might be incompatible with IMBlocker