The block heater in your car won't do anything for you as far as fuel goes. It's purpose is to heat the anti-freeze in engine so that it doesn't freeze (which is very possible up your way!) and damage the engine block and to make it easier to start up the car on cold mornings.
As far as heating the fuel itself - You have to consider your setup. There are two ways to going biodiesel (ok, there are several but they fall into one or the other category here..). If you are making bio-diesel in your garage or you are buying it from a retail pump then it should perform about the same as #2 diesel you currently buy. In very cold climates it's not uncommon for diesels to have a fuel heater to prevent the fuel from jelling and plugging the filters (technically, that is referred to as "cold flow plugging".
). If you don't have a fuel heater you could get one fairly easily.
There is another technique where your fuel tank is used to store raw veggie oil and it is mixed within the car itself as you drive. IMO, this is less efficient but this setup requires an in-car "boiler" to heat up the mixture. I don't know how they prevent the unprocessed oils in the tank from jelling in this setup.
As far as how it burns in comparison to #2 diesel - B100 (100% bio-diesel) provides 92% of the energy of #2 diesel so you are most likely going to see a slight loss in your miles/gallon. B20 (20% Bio-diesel mixed with 80% #2 diesel) has 98% the energy of straight #2 diesel so you could see a minute drop there as well. It's up to you to decide if the loss in milage is woth the decreased dependency on fossil fuels and the environmental impacts.