At the high altitudes and speeds the Blackbird would have to go to evade a MiG-25, it would have no hope at all of evading a missile.
I thought the SR-71 was faster than missiles?
https://nationalinterest.org/blog/buzz/forget-stealth%E2%80%...
> In effect, by the time it took a SAM system could lock onto an SR-71 traveling at Mach 3 and launch a missile, the Blackbird was already moving beyond the effective range of the missile.
It wasn't purely faster than the missiles, it was fast enough to get out of range of radar and tracking for ground based missiles and beyond the effective range of the missile before it could reach the jet. Fired from an already moving jet at altitude missiles would have a longer effective range as it would spend less fuel getting to speed and gaining altitude.
The Soviet Union at the time had the R-40 series of air-to-air missiles, that could got Mach 4.5 at very high altitudes if launched by a fast enough, high altitude aircraft, which is much higher than the Mach 3.4 the SR-71 could hope to achieve. The reason it could go that fast is because it didn't have to expend its kinetic energy to climb much, and because it already started up at Mach 3.