This is a study from José Luis Nieto Amada called "La biolantropologia del Valle del Ebro" (bioanthropology of the Valley of Ebro) from 1983. The studied populations are the valley of Ebro, that is, from the provinces of Navarra, Rioja, Huesca, Zaragoza, Teruel and part of Lleida. That is, in the Northeast area of Spain.
Here are some quotes from the study :
"In the lands of Ebro, there is a predominance of dark chestnut hair color".
"According to Sánchez Fernández, Zaragoza would be the province with the highest percentage of blondism, with more than 30%. The percentages of Huesca and La Rioja would also be among the highest "
"Our statistics are far from these frequencies, and we place in the Ribera Zaragozona and Navarra (20.5%) the highest concentration of blondism. "
"In our study, the highest frequencies of blue eyes appear in the communities with also the highes percentage of azure iris. The distribution is very homogeneus between navarrese and aragonese, with a marked difference in the Ribera (21%) and the middle basin of Jiloca. Wheras in Rioja, the percentage is much lower."