My memory is that when grayed out Chicago was right on the border of legibility, but I found this image [0] in another HN post, and it's slightly better than I remembered.
[0]: http://www.computerhistory.org/atchm//wp-content/uploads/2013/01/screen_image_filters.jpg
The original Mac had no color or gray scale; only one bit per pixel. Disabled items in menus were indicated by a dithered gray, a simple on-off alternating pattern.
Thus, Chicago had to be designed to be legible even when every other pixel was removed. It really was a 12pt masterpiece.