Getting additional S's after getting M7 or while maxed on M6/7 Tokens should give you blue essence
I have like 4 M7s ready to be crafted but absolutely no blue essence. Not to mention that every time I actually manage to save up the 1700 for the champ shard its usually
or
for some reason. I think giving BE for additional S scores would be a step in the right direction for improving the satisfaction of the Mastery system.