Dude, I am pretty sure the left/blue one is the low temp intercooler coolant tank. My lower center radiator/intercooler radiator hit a rock yesterday, and it sprayed coolant out through the grill. Right after I stopped the car, the coolant from here is like my body temp. After like 2 hours, I checked the Left/Blue coolant tank, it was empty.
http://m5carblog.blogspot.com/2013/02/cooling.html
This post shows the turbo water cooling line is connected to the engine cooling circuit, and the low temp coolant line only for DME and intercooler
I never know about my car until bad things happen, but its a little expensive to learn.
Taking my car to the dealer tomorrow