What is the opposite of "black mark"?

Solution 1:

Rather than gold star, I suggest plus. Gold star would be awkward here:

It was another plus for her that she had attended the meeting.

Solution 2:

Gold Star

Informal.
Symbolic approval or recognition for outstanding merit or effort:
You get the gold star for cooking such a gourmet dinner.
anything that represents an outstanding effort or achievement:
Her promotion was the gold star she'd been working for.
http://dictionary.reference.com/browse/gold+star

Both terms conjure up images of a teacher's gradebook, with either a black mark indicating a discipline or academic problem, or a gold tinfoil star sticker representing extraordinary achievement. This also maintains the pattern of a color followed by the mark.

Solution 3:

Full marks (definition: full credit for an achievement; praise that someone deserves for accomplishment, hard work, intelligence or other quality).

Or expression of satisfaction (example: For our wounded Your Majesty's expression of satisfaction is going to become a source of consolation).

And, of course, some "brownie points" for @Mitch comment: gold star.