I'm currently also looking into yarn for a blanket, basically with the same requirements as you have. I want a warm and heavy blanket for the cold winter days.

The kntting store recommended Boston by Schachenmayr to me which I found a bit too scartchy for my taste, although it does come in some nice colours and the price is relatively reasonable. Personally I'm tempted more by Merino Big by Schachenmayr which is a bit softer.

Last year I knitted a blanket for my mom and of course now I no longer remember what yarn I used. Hm, it might have been Vario by Lana Grossa.

Hope that helps :_)