What material was/is used on the skin of the Blackbird SR71 and how was it cooled. How was it manufactured to be stronger with age
Titanium. Only the hottest portions of the fuselage (the chines) were cooled, using the fuel as a coolant. As far as I'm aware, there was no treatment to make the material stronger with age.
Titanium is the only suitable material which can withstand those temperatures. Stainless steel is much heavier. If you want to find out more about this read Ben Rich's book on the Skunk Works.