I’ve also seen ⅆ as the differential operator and 𝔼(X) as the expected value of X. I think I’ve also seen 𝟙_x, and ⅈ, and ⅉ but I don’t remember for what.
I know 𝟙_X as the indicator function (also called characteristic function) of X in measure theory and probability theory, but I’ve also seen 1_X or χ_X.
ⅈ and ⅉ can be basis vectors or imaginary units although I’ve never seen them in blackboard bold in my lectures.