What are different types of dimmers?
Without knowing specifics of the electric blanket, and what it contains as internal electronics, it is difficult to provide a definitive answer. 

The simplistic assumption I was tempted to make, that the electric blanket essentially has just a bunch of resistances (wires) as heating elements, is unsubstantiated:
That kind of design could be potentially fatal if one were to damage the blanket and touch mains-voltage heaters!

If one were to assume that whatever electronics the blanket integrates, is compatible with a varying partial sine wave mains supply, then yes, a triac dimmer of sufficient power rating would be the way to go. 

If not, a simple alternative would be to implement a dimmer consisting of a suitably rated solid-state relay controlled by some timer circuit, to switch the blanket on and off every few minutes, with the on-time percentage varying according to desired temperature. 

The reason this would work is:
An electric blanket (under typical use cases) does not need its temperature precisely controlled on a continuous basis. The heating itself takes many minutes, so a power cycling span of minutes would be acceptable to even out the temperature to a desired set-point.

The SSR is suggested instead of a conventional electromagnetic relay, because it would operate silently, and also would last longer due to not having parts prone to mechanical wear or contact arcing.