The SR-71 engine is a weird beast operating like both a ramjet and turbojet due shaping the flow and injecting fuel in the air stream.
A ramjet [1] stays efficient at high speeds even though it on the outside kind of looks like an afterburner.
It was a conventional afterburning turbojet for take-off and acceleration to Mach 2 and then used permanent compressor bleed to the afterburner above Mach 2. The way the engine worked at cruise led it to be described as "acting like a turboramjet".
The afterburner on the SR-71's engine is being fed super-compressed bypass air from the intake. It can ignite the exhaust with comparatively minuscule amounts of fuel when compared to a regular turbojet.
At speeds beyond Mach 3, you don't even need fuel to ignite the oxygen. The simple friction and drag of the airframe is enough to ignite the oxygen around it and surround the aircraft with superheated plasma.
Edit: Of course, the Blackbird had the benefit of refuelling mid-air.
[1] https://youtu.be/gkyVZxtsubM