When you say 'blend fragrances', what do you mean?
Can't think of anywhere where they will do a custom blend for you. Done well, this would be extremely expensive. Le Labo blend each bottle for the client at point of purchase, although you cannot request a custom blend. They have a concession at Liberty and a store in Marylebone.
Les Scenteurs, again Marylebone area, are very good. Lots of different perfume houses and very knowledgeable. You can request samples, which helps.
Selfridges is very good for perfume, but you are limited in that the sales assistants are brand specific, so there is no one person who is able to advise across all brands, if you see what I mean. But they stock Chanel Les Exclusifs, and the full Hermes range including the Hermessence range, which Les Scenteurs don't. Liberty is also very good - Le Labo, Frederic Malle, Byredo, as well as a perfume room which has niche lines like CB I Hate Perfume (Christopher Brosius WILL do you a custom blend, but he is in New York, there is a long waiting list, and it costs thousands).
The Perfume Room on the 4th floor in Harrods is amazing with extremely knowledgeable staff. Very high end and very expensive lines though. I'm not sure whether Roja Dove at Harrods does custom blends. Again, will be hugely expensive.
I'm afraid I only know London, which might not be at all helpful to you. Les Scenteurs have an online service and you can purchase samples.