Some things that might explain the intense blushing:

a) anxiety is comorbid with ADHD. What this means is that if they do have ADHD/ADD then is it highly likely they also have some sort of generalized anxiety.

b) if this person is amab then any sort of compliment is going to send them over the moon. amab individuals rarely get the same romantic attention that afab's do. So it's entirely possible they simply didn't know how to respond to that situation (I still remember the first time a person complimented me)

All of this is to say it sounds like you have a good handle on the situation but you might want to consider slowly ramping up the obviousness of your intents and watch how they respond. Obviously do whatever you are comfortable with, but it is probably a good idea to try and take the lead in this situation (but again, trust your instincts)