License for using background music in my video?
Having licensed music for videos I've produced; from having custom tracks re-recorded by one of Audio Jungles top artists (1 Million in Sales); to one hit wonders who had radio airplay; to even Katy Perry... 

You need to find out what Label the album is under; meaning, who owns the rights. 

Blackmill will have no say, unless they own the masters which I doubt. 

The label information should be easilly found on the CD/Album info. 

Contact the label; and they will either have a licensing department or they will refer you to their agency. 

It is a time consuming process; there is a lot of back and forth, and it could be expensive. Very. If they are under a large label; and they don't have an agreement with a "licencing agency" like ExtremeMusic; which basically mass sells known artists and because they do so on a large scale basis they have a basic rate sheet; 1 song for x use for x type of duplication used in x such a way = this price;  if they don't have that track on a easy to buy list; it's going to involve a custom negotiation where money talks and if you don't have the money...  don't expect a return call. 

Contacting Blackmill or even their manager/management is likely going to prove a waste of time.