The Blubberwing is only mentioned in How to Ride a Dragon's Storm.
There is no physical description of the Blubberwing Dragon, only its product: Blubberwing fat. Assuming this dragon has a lot of blubber, most likely it is fairly sizeable and rotund.
Blubber is stored fat in the body and provides the animal with food stores as well as insulation from frigid weather. It is this aspect that the Vikings are utilizing. The Vikings will smear Blubberwing fat on their bodies to protect them from the cold seas.
|“||Don't be a softy!" roared Gobber. "The Blubberwing fat you have rubbed all over you SHOULD keep you warm enough to prevent actual DEATH ... but its all part of the game of course.||”|
|— Gobber the Belch|
Specifically, Gobber the Belch is referring to the novices of the Hairy Hooligan Tribe contenders of the Inter-Tribal Swimming Race. Later, Dogsbreath the Duhbrain dumps a "pot of slimy green Blubberwing goo" all over Fishlegs.