I never "blow by" unless the person just "blew by" me. Call it trading blows.
To me, the safe distance is 3-4 feet on their left, ease up into the "awareness" area with a friendly word or two, compliment their bike, and slowly go by. 30-45 seconds is not too long to wait until there's a chance to pass safely, it just seems longer out on the road. And sometimes, like on the same group ride roccobike refers to, you come alongside a guy on a 1983 Centurion Elite GT, and you spend a few miles in conversation.
If the rider is in a aero tuck, there's a slight advantage because they are more likely to maintain their line, especially on a straightaway. If the rider is aero approaching a corner, I hang back because you just don't know when he/she will untuck and corner, or go straight. They rarely signal.
If it's a pace line in aero tuck, I hold back, approach slowly on the left a good 6' away (outside tumbling carbon fiber range) and say "looking good, guys." This feeds that ego thing, gives them a chance to rationalize why the steel solo rider is passing them, and you have a short period of inactivity in the pace line when you can generally go by. Of course, they will very likely catch you; I think it's written somewhere that they have to show you they can. By far, the most dangerous place to approach and pass an aero tucked pace line is descending, so I don't. Climbing is easier. As you go by, if you do, they will all look at your chain ring and rear cassette, to see if you're pushing whatever they're pushing. That gives you the chance to nod and move on, and face it, passing while climbing is not exactly a quick thing.