Sandblasting dark spots left

[MX145]

On my 68 Camaro areas that originally had a little more than surface rust have dark spots after sandblasting. The rest of the car turned out great with the white/grey clean metal color. Do I need to keep sandblasting until the spots go away or use some type of rust conversion process before epoxy primer? I'm trying to avoid using a rust sealer product like POR-15 because I don't want to get the car wet with their metal clean and prep process while I'm working some other bare metal areas. Seems like a lot of pro builders use epoxy right over the media blasted surface and I want to make sure I do this correctly. The spots show up in areas behind the quarters and inner roof structure so they won't need finish paint work. What do you guys think?