Small Blue (Cupido minimus)
Whilst it is small it is not blue however! This is the male which has just a hint of blue on a charcoal background whereas the female lacks the hint of blue altogether. It has no other markings on the upper side of the wing but does have the light coloured border like most of the 'blue' family.
Small Blues are hard to see for a variety of reasons. Firstly, their size means they are easily over looked. Then they have a very short season flying for a couple of weeks around the end of May and beginning of June. There is a partial second brood towards the end of August.
Thirdly, they are quite rare, found in very few locations and in Dorset we have small colonies at Durlston and on Portland. (Maybe elsewhere but I am not aware of any?)
Finally, they live in little pockets just a few yards across; they favour small patches where Kidney Vetch grows. It is possible to be in the right place at the right time and yet miss them because they are so limited in range.
Going looking for the Small Blue? The very best of luck!