This looks like a poem to me. Apart from the あぁ at the start, it's in a 7-7-7 moraic configuration, which suggests the writer is adhering to a particular scheme. If this is a poem, the usual rules of grammar don't apply. There are a lot of ways in which poets 'bend the rules' of grammar in order to keep to the standardized form. So I wouldn't be too worried if something looks a little out of place sometimes.
Having said that, the が here is just pointing out the subject of a verb, which is 花火. The verb is 咲く, ie 花火が咲く. However, it's also possible that another verb is omitted in the first phrase, like the 花火が夜空を彩る I mentioned in my comment. Then 咲く is used again with 花火 still as the subject.
In either case, the が is just the normal subject marker.