Not to defend any notion that black people were not present in Spain before the 1970s, but regarding the term 'Moor', it's not necessarily as clear-cut as how you imply though, since the term is not clearly defined and has changed considerably throughout history. The 'original' Moors (as in the muslim population of the Al-Andalus empire) could have been different than those called 'Moor' in the times of the slave trade. The original Moors are generally considered to be composed of Berber and Arabian people. Genetic analysis of the Berber people shows that at some point in time, there must have been quite some variation in skin colour in these people, ranging from fair skinned (even blond and red-haired) to really dark skinned. The Berber were considered to span an area from the Mediterranian to Mali and Niger, so that would make sense. Whether this was also still the case in Medieval times, I don't know. The whole of Northern Africa (which is considered to have been populated originally by much darker skinned people than nowadays, as far as I know) had been integrated in the Roman Empire for centuries before and by Arabs later, which might have changed the demographics of the Berber people by then.
The Moors depicted in the paintings above could have come from anywhere, as in those days muslims from any region of the world (India, Philippines, Africa, etc.) were also called Moors.
So, in short, it's definately possible that there was a big variation in skin colour in Spain between the 9th and 14th century. But it's also possible that the population looked more homogenously like the current inhabitants of North-Africa. I'm not sure if there are any historical sources on that.