Shame you didn't have a photo showing the undersides...
As it is, it is not clear-cut to me...
Note that with many species of blue, the females are partly or totally brown; it is the males that are the bluest.
My best guess is black-eyed blue, Glaucopsyche melanops algirica.
But there are some problems, eg. the flight period is April-May (not later than June according to http://www.bulletinofinsectology.org/pdfarticles/vol63-2010-161-170lopez-villalta.pdf)
Others with some points in common with yours:
- Holly blue, Celastrina argiolus (various aspects of colouring are wrong, eg. width of dark border to hind wing)
- Little blue, Cupido minimus (but yours is too blue, other things)
- Lorquin's blue, Cupido lorquinii (but that flies in early summer, mainly in Morocco and the Sierra Nevada, not Extremadura) http://www.butterfly-guide.co.uk/species/blues/andal1.htm
Here's a list of butterflies of Extremadura
Anybody else got some insight?
Ah - that helps - thanks.
I am pretty sure it is an African grass blue, Zizeeria knysna (also called Zizeeria karsandra)