Pressing and holding 'j', and wanting to check the condition of that key every single time the macro loops through, is going to make for a slow and painful macro, and one potentially fraught with timing errors.
If I were wanting to do something like this, well, I wouldn't use a single-key trigger :). But If I absolutely had to, I would use it as a marker: Press 'b' to denote the start of the bold, move the cursor to where you want the bold to end, press 'b' again. The macro then does the actual work of bolding the text.
But honestly, I still wouldn't do it that way. If I wanted to use a single key, I'd use a single modifier key, because that's not sent to any app. And that's exactly what I did when I wrote something similar, my Universal Highlighter macro, which uses the Highlight feature in 10+ apps to let you quickly highlight text, continuously, by just selecting chunks of text.
You launch it with a modifier key double-tap, then it keeps running until you tap any modifier key once. While running, as you select text, it's highlighted as soon as you release the mouse button.
You could use a structure like this (which requires three "Until" loops to track keys and the mouse) to bold instead of highlight. And if it was just for one particular app, it could be a lot simpler—mine kept expanding to cover more and more apps that offer highlighting.
-rob.