Dark Leaved Willow. Salix Myrsinifolia. Formerly known as Salix Nigricans
Here are some websites which help with identifying and describing Dark Leaved Willow. I have included some non-Scottish websites because they have good descriptions of this tree. It is apparently very similar to the tea-leaved willow. There does not seem to be a great deal of information on the web about this particular variety. I have sofar found only two pages dedicated to the species - see the iframe below. However, there is information about dark leaved willow on the Yorkshire Willows website, and there is a picture there.