Trouble with PB blaster is when there's simply no space for it to even get into the affected area. Ideally you would be able to soak the entire seat tube in PB, but that's pretty difficult.
You could try this: If you have enough seat post exposed, try to knock the post (with a 5 lb sledge on a wood block and the frame resting on wood or some sort of padding softer than concrete) deeper into the frame. The idea being that you might break the bond between the post and the tube. Heat and cold sometimes help, but you could really damage the paint and just find yourself more frustrated.