Originally Posted by
Machaca
Because [Concorde] was optimized for M2, and Blackbird for M3.
Indeed - though the difference in design brief is considerably greater once you get into specifics.
Concorde was designed to "supercruise" at M2 (i.e. without reheat/afterburner) because it was a hundred-seater airliner intended to provide supersonic service on the transatlantic and (initially) some of the old "Empire" routes. It had to use regular Jet A-1 (or equivalent) and the presence of passengers and luggage meant there was limited fuel capacity. Because of all these factors using reheat to sustain M2 was out of the question. The genius in Concorde's design was not so much in the speeds achieved as it was the way in which such speeds could be achieved and maintained over a long distance - with relatively remarkable fuel economy.
The SR-71's design brief was relatively straightforward - make it as fast as possible so that SA missiles of the type that downed Gary Powers' U-2 could not be a threat. Its payload was a pilot and high-tech imaging apparatus, meaning that it could also carry a boatload of fuel. Being the height of the Cold War, no expense was spared - even a type-specific fuel (JP-7) was developed for the project. As far as the propulsion was concerned, only a ramjet design was capable of sustaining M3, hence having the burners lit was a cornerstone of the brief from the beginning. The genius in this design was all about the speed, however special mention should be made of how pre-existing technology was leveraged and modified in a very economical way.
Apologies for the newbie talk on a thread that clearly has gone into the "expert" realm!