I think it is easy to imagine circumstances where what you call a 'blanket' search - a search for specific categories of evidence, but which isn't limited by particular approved keywords - would be justified. I would expect it to be common, because I would expect it to be the only reasonable way to conduct the search in a great many cases.
I am a litigator. We regularly conduct similar searches during what Americans would call discovery - the pre-trial process where you look for evidence. Even if you know exactly what sort of thing you're looking for and are using specialist software designed for this process, it can take hundreds of man-hours and cost well over a hundred thousand dollars to search a large body of correspondence. I can tell you from experience that it is very difficult to pick keywords in advance without making them incredibly broad. If the person may have done something that they don't want to set out explicitly in an email, it can easily be impossible.
There is a strong public interest in effectively investigating crimes and bringing the perpetrators to justice. There has to be some mechanism to get access to email correspondence in order to conduct that investigation. In a big investigation, I can't see that it would be reasonable to require Google to carry out the searches required - it should be done by the police. I therefore think it is entirely plausible that a 'blanket' search warrant would be granted, and I haven't seen any reason not to trust the judge's assessment - after all, he has heard the evidence and you haven't.