A girl in a purple top, baggy pants and sneakers is blowing a kiss, while another girl in black watches.
A girl blows a kiss as another girl sees it.
"Is blowing" is a rephrasing of "blows", and if another girl watches, then surely she sees it.
Girl in a purple top is a type of girl, and blowing a kiss is a paraphrase of blows a kiss.
A girl blowing a kiss , while another girl watches her.