Wescott wrote: If I were a better barista, I might have a keen perception of blonding in all its incarnations.
Blonding is an indication that the machine is starting to pull flavors that you find less appealing. The real way to find out when to stop the shot is to divide the shot into five or six parts, using some visual stimuli to keep track of what part is what (first drops, single cone forms, cone grows, flecks stop, striping gets pale, cone wanes, striping stops... whatever) and taste each part. If they taste good all the way to 'blonde' whatever you perceive that to be, keep them all as part of your daily cup. If you find that after 'x' they start to taste awful, this is your chance to describe to yourself when to stop in terms you're familiar with, and excise that taste from your routine.