Un interessant article paru sur Quora :
The premise of the question is wrong, and I’m not sure what could have driven the author to ask it given that it’s generally known in Spain that darker features are more common along the Mediterranean coast.
In fact, Valencia and the south of Catalonia are some of the regions with the least people with blonde hair.
Same with regards to blue (or green) eyes. Valencia and a good chunk of Catalonia have darker eyes than the rest of the Spanish population.
So, to answer the spirit of the question, the reason why blonde hair and blue eyes are less common in Catalonia, Valencia and other regions along the Mediterranean is that, historically speaking, these regions have had more exchange with other Mediterranean peoples and little to no contact with the Celtic populations in the center, north and west of the Iberian peninsula.