I had the same curiosity about offering a blank keyboard because I've been using one for 8 years or so after it was recommended by a former co-worker who I have huge respect for. So I'll give it a shot to explain the difference after I got adjusted to it:
I did not realize that my brain was still wired to look at the keyboard periodically, specifically when dealing with special characters and really anything outside of typical alphanumeric. I only realized after the adjustment that each time I had to find one of these keys it was a minor interruption to my current thoughts. The best way to describe it is imagine a piano with every single key labeled as to what the note is. It would be difficult to teach people to think about the creativity and harmony and melody aspect of music and to directly translate what they hear in their head into muscle memory to reproduce it. With the keys labeled it would be like training wheels and difficult to get people off of this because the translation would constantly be:
- I hear a "G" in my head
- I see a "G" on the piano
- I play the "G" on the piano
Instead really what you want the brain to do is say:
- Here's what I play to get the sound in my head
With no in between. Or similar to learning a new language and having to internally translate things to your native language before you can understand (or the reverse for speaking). Instead you just want to immediately know what this language is saying with no native translation.
TL,DR: Once you really internalize that looking at the keyboard makes 0 difference, it's one less minor thing for your brain to think about and your flow isn't interrupted as frequent by internal thoughts. Using a blank keyboard is just a (admittedly somewhat extreme) way to force yourself into this.