I might just have to out this lot as a warning. Looks like they messed up.
No, you shouldn't. There was already something wrong with the frame and it doesn't have a damn thing to do with the shot blasting. Is the media/method they used a bit too aggressive? Probably, but there may have been a reason. Was the previous finish paint or powdercoat? Sometimes using less abrasive media can take forever if the finish is difficult to remove, should they spend hours on your frame and charge you for the extra time? I doubt you would be happy with that either. Is it a shop that does bicycles regularly? That makes a huge difference, but they usually charge more because it is more detailed than just stripping a hunk of cast iron. If you went to the cheapest place that you could find, you probably got what you paid for.
You are not suddenly an expert because you've read 10 responses on Retrobike. Publicly badmouthing someone's business is not something to be taken lightly.
I agree with you, there was an underlying problem with the frame, I suspect it was probably weakened by the seatpost. So perhaps they did me a favour by revealing something that would have come back to bite me.
I went to a company that has been recommended on this site, cost was not a factor in my choice.
I didn't mean the 'outing comment' seriously, which is why I haven't done it. But I do think if they have made an error in the way they blasted the frame, i.e with too much force (what's your opinion of the close up photos?), then someone who takes their treasured frame to them should be aware of it, what is the consensus on that?
You are also right to say I am not an expert, which was the point of the post, there are people here who can advise me, and that's what I asked for.