A bolster pudding is any sort of pudding made with suet-based dough spread with jam and rolled-up to look like a bolster cushion. These days these are usually called a roly poly or simply a jam roll, and are usually baked. The bolster pudding was not baked, but wrapped in cloth and either boiled or steamed, usually for a period of a couple of hours.
Bolster puddings had a number of names, including Dog in a Blanket and Jack in a Blanket, refering to the appearance of someone rolled in a blanket. Even less savory, they were sometimes called 'shirt-sleeve puddings' or 'stocking pudding' as old bits of clothing were often repurposed to the task of cooking the puddings. The use of old clothing as forms is probably also the origin of the names Dead Man's Arm and Dead Man's Leg.
The filling was variable; black currant appears to have been the most often referenced, but any sort of jam or fruit preserves could be used. In America, a 1864 recipe for a type of bolster pudding called Bacon Pudding used apples and onions in place of berry-based jams, and added plenty of bacon for flavor.
"Jam Pudding: Ingredients, one pound of flour, six ounces of suet, half a pint of water, a pinch of salt, one pound of any kind of common jam. Mix the flour, suet, water, and salt into a firm, compact kind of paste; roll this out with a rolling-pin, sprinkling some flour on the table to prevent the paste from sticking to either; fold up the paste, and roll it out again; repeat the rolling-out and folding three times; this operation will make the paste lighter. Next, roll out the paste one foot long by eighteen inches wide, spread the jam all over this, roll up the pudding in the form of a bolster, roll it up in a well-greased and floured cloth, tie it up tightly at both ends; put the pudding into a pot of boiling water, and boil it for nearly two hours; when done, turn out carefully on to its dish, without breaking the crust."
-- A Plain Cookery Book for the Working Classes by Charles Elmé Francatelli (1846), sourced from Foods Of England.