They don't say exactly what gets you there. However, there are three elements which seem to matter:
1. The ad must have been reviewed (so, an ad typically CAN'T run in the blue box for the first 3-5 days)
2. The ad must have a high CTR. "high CTR" means high CTR for the keywords, as this will vary from keyword to keyword. so, there's no particular CTR. It has to be higher than the others on the page, and perhaps significantly higher.
3. There must be a number of ads on that page. This seems the most nebulous of the rules, as I HAVE occasionally seen a blue box with one or two ads, and no other ads on the page. But, generally, you won't see one when there are only a small number of ads.
I'm sure there are exceptions to this rule, but if the terms you are bidding on do not have anyone in the top horizontal spot - it's because they are not 'popular' enough or lucrative enough for Google to run them.