During the summer months I use a Vango Venom 200 Square Sleeping Bag, which can be fully unzipped to form a down blanket.

http://www.outdooraction.co.uk/acatalog/lightweight_Vango_Venom_200_Square_Sleeping_Bag_.html

It is used in conjuction with a Thermarest neoair short sleeping pad. If used with a lightweight bivvy bag the combination will still weigh less than a Wool blanket (I'm assuming around 1.6 Kg) and will easily outperform it for sleep comfort in colder weather conditions.