Well, for what I see Charlton Heston is descended from Scots and English, so British. Do you know who were the first people to walk the British island? Proto-basques. And the celts than populated the Ireland, the Milesians? Spaniards too, from Galicia. So yeah, I don't see that far-fetched for this guy to play El-Cid.
Oh, and both Spain and the british island were invaded be Germanics tribes (The alans, Visigoths, Vandals and Suebi in Spain, Jutes, Danes, Angles, Saxons etc in the British Islands), so yeah, Spain has natural blondes too.
And about Moor blood and all, being nearly all the muslims converted natives, the actions of the Inquisitors, expelling people and all that made the blood impact very low. Heck, Spain has a lot more Jewish blood than moorish if you see the DNA tests.