I cannot reproduce the code here as I don't know how to type 'e' in the blackboard font. I typed it in Mathematica by using the Esc key.
The problem is that in my input code, I've got an upright 'e' while in the plot legends, I've got an italic 'e'. Why does this happen and how to get an upright 'e' in the legend as well? I'm using Mathematica 188.8.131.52.