How does Blockstack solve the problem of learning the PGP key of your friend if all you know is his email address ?
If email is not the lookup key, then there can be more than 1 profile matching this email. Which one do you choose?
Alternatively, if email is the lookup key, and Alice could register an md5 of her email, call it H, as H.id, then Alice would screwed if someone registered that before her (trying to impersonate her). Would you propose in this case that Alice would form a hash H = H(email + "1"), which would make the hash unique? Yet, if she does so, how would his friend Bob know to add "1" to his email without communication with Alice ?
And if Bob must must communicate with Alice securely before the lookup could be done, then what is the point of the lookup ? Then Alice can just tell Bob her key.