I am not that concerned about the Gothic style dress, phone behavior and overall "metalhead" appearance that she has taken on. But I am concerned that she assumed this personality because a boy took away her self-worth with a comment that she was "ugly" and then gave it back to her when she became whom he wanted her to be. That's a very destructive message for any youngster.
At 10, she should not be "going out" with anyone. She also should not be going to the mall on her own. You need to set and enforce some rules and limits with her in these two areas. She is too young to be on her own like this. Make sure that you tell her why she cannot continue to go out with this boy and be at the mall on her own. Mention the concerns and values that shape your rules and limits in these areas. I would also recommend inviting this boy's parents over for a chat.
At 10 years old, kids are usually involved in best friend relationships, not in exclusive boyfriend-girlfriend relationships. Could she be imitating an older sibling or relative with this dating relationship? She has plenty of time before dating becomes healthy developmentally. For now, you need to hold your ground on these prohibitions and take the flak from her for "ruining her life." Expect an angry, possibly rebellious 10 year old on your hands but don't give in to the "but all the other kids do it" ploy.