Disclaimer it was 3 in the morning and now is 2am again, but it was bothering me.
https://v2.steemconnect.com/docs/steemjs#api/get_reblogged_by
Basic Code
And basic results. Sorry not sure how the fuck I missed this, their api does the work for you, not sure how but a simple steem.api.getRebloggedBy will return all reblogged.
I hope this helps. classic case of RTFM haha.