Mina does not seem to be overly concerned by the "something, long and black" which she sees bent over Lucy at the seat. Why do you think this is? What do you Mina thought she saw?

I wouldn't say she wasn't overly concerned. Mina seemed to move as quickly as she could through the dark shadows of the night. There is no evidence to say who she thought the dark figure was. They see "a man, who seemed not quite sober, passing along a street in front of us" but there is no evidence to say she thought the man was the dark figure in question.