There are a number of blue ranks that have highly stable performance, with a set of well executed and robust game plans that result in high army efficiency. The strength comes from, good plan, good evaluation of tactical situation, unit micro and sense for right transition timing, even though the plan itself is pretty obvious after seeing it a few times.
I don't think there is a consistent game plan that gets you to purple and it is more hit or miss. The kind of things purples do beyond blues, from what I've seen are:
1. Opening gambits that quickly collapses weaker fronts, resulting in fast advantage
2. Unreliable Gambles that saves a game from losing situation: the best athena backstabs I've seen came from purples. There are also things like sniping paladins, shield or lance balls with bombs across a thick screen. Some purples can also sniff out this and prevent it, for example thinking about weird silo locations that generally is not looked at.
3. Play effectively with large and weird comp. armies from leavers and sometimes grind out a win afterwards.
4. Effective rear guard action against heavily stacked front on maps with isolated lanes (most stable game plans require not absurd econ balance)
5. Effective play in less common map types: sea maps, hill maps, very wide front maps, larva map adaptations
6. Weird compositions that sometimes is effective against whatever it is used against, but doesn't work if someone just wants to copy it. (probably situational with most players not having enough game experience to know counter gameplay/transitions works)
7. More messages to other players on what to do. This is kinda annoying but purple automatically gives weight to suggestions. This can be decisive in things like strider rushes or superweapon/nuke/econ strategy beyond contribution from normal individual gameplay.
8. Major army position changes: entire lance army shift across maps between inaccessible terrain with charon? That kind of thing.
That said, I do suspect many casual purples come from 1v1 and FFA. Some can be identified with 1v1 style composition/gameplan as opposed to pot-type game plans, not very common sight in pots and causing their pot team to lose more often than not.
If a purple gets into an tactical fight with high ranked blue they probably not really carry, but when some lower ranks stall out blues with defensive play (for example dirtbags) maybe a way of winning can be found.