Blue rate can't go higher because of the harsh controls on the cuevas. It's not a supply and demand problem as much as it is a comply or die kind of thing.
As wongjoh mentioned, the cuevas were attacked pretty seriously by the government. The cuevas kept letting the blue rate go up even though the government had been trying to stop the climb for at least a year maybe more by threatening them. After lead players were arrested, a bank lost its license to operate and surely other things, the cuevas made a deal with the government to keep the price down, or continue to get harassed, arrested and shut down. Every time things started to move up a little bit since then, the government has raided arbolitos that I'd never seen raided before and shut them down for a day. The one where I usually do business has been shut down 3 times in the last month and a half or so, always as the price was passing more than 10 or 20 centavos over 13. It's artificial now, and I don't think we're going to see anything much higher than 13 before Cristina's out of office, or something happens that effectively makes it difficult or impossible for Cristina to continue to harass the blue players.
Let's hope whoever inherits power will devalue the peso and give some slack to the blue rate until the official rate catches up. But I have no idea what the next two years are going to look like here.