I recently had a filling about 10 weeks ago and the amalgam in it has turned black, is that usual?

Depends: The ph (acidity or alkalinity) of your saliva due to various systemic factors or diet may cause this to occur. I do not like amalgam fillings because teeth often crack or fracture due to a difference between how the tooth and filling react to temperature changes (hot & cold foods).

Yes: They often do turn dark. Maybe partially due to a roughness that can polish up at least 24 hrs after it is placed. Ask your dentist that placed it. Amalgam will also cast dark shadows in your teeth and even permanently precipitate stain inside your teeth. It is a very tough material though and when used correctly is probably the most durable option short of more expensive options like crowns.