I find the current Storm gorgeous and think she's perfectly fine the way she is. However, if she were to take up tanning on a regular basis, you would definitely see tan lines if she didn't do so in the nude.
Also, the point of a black person tanning is not to "become fully black", it's just like with white people, some just enjoy laying out in the sun, and some just feel sexier with darker skin tones, or other reasons.
As for what he was getting at, I cannot, a hundred percent be sure, but there are many, many people who don't realize that black people can get tanned. I was merely just explaining that yes, they can. Of course, it could just be that he thinks Storm with tan lines wouldn't be as attractive as the current, untanned Storm.