On Windows and Linux, it's AltShiftA, as RichieHindle pointed out. On OSX it's OptionCommandA (??A). It's also worth noting that the two modes can have different font preferences, so if you've changed the default text font, it can be jarring to toggle block selection modes and see the font change.
Finally, the "search commands" (Ctrl3 or Command3) pop-up will find it for you if you type block. This is useful if you use the feature just frequently enough to forget the hotkey.