We never really get told that Klingons would be particularly strong. Yes, they endure beating, and TNG "Ethics" establishes redundant systems that give a biological advantage wrt combat wounds - but muscular strength has never been something the Klingons are known for. It's just "warrior fierceness".
As pointed out on occasion, in ST3, Kruge enjoys demonstrating his personal strength. But that quite possibly indicates he is an exception, which is the very reason he enjoys the demonstrations in front of his fellow Klingons.
Purple blood never reappears after ST6, that's true. Perhaps it only turned pink because of the type of weapon that hit Gorkon - and the reason the blood of the masked Starfleet assassin at the end was immediately recognized as non-Klingon was not the color, but some other attribute?
Or perhaps Gorkon was so drunk or high on another substance of his preference that his blood chemistry exhibited a color change?