I don't think that array ever gets used, if you look on steemd they also always have an empty reblogged array, I can't be sure but I think this is up to the implementing app to fill in, only one I found that does this is esteem app , I have not seen posts created by busy, steemit, dmania etc have this filled in , this is just from comparing a few blog results with feed results to try and find a connection.
I think most apps only know you resteemed something since they maybe compare the author with the account name , no match = resteem.
It seems also that you will not know who resteemed it since there will only ever be a single origin of the post on the blockchain, apart from having a relational index that maybe could compare all feeds that contain that permalink or post id I do not see how you would know. A very costly query.
I might be wrong but poking around for 30min this is my conclusion.
I've been poking around too, and it seems as though this isn't being used.
I did wonder if whatever database (or whatever) that is used to generate the json file has the data, but then in the query that creates the json object, the SQL is malformed somehow resulting in an empty array.
I think I found something. If you view according to account history you can see the action transactions, reblogging is a custom json transaction that falls under follow. will update once I can confirm. haha just wanted to reply it seemed a bit exciting.
edit: nvm @sambillingham posted a nice explanation of what I found also.
I read something about reading through the custom json transactions to find data. That would be nice if the reblogged_by could be retrieved this way.