They can be used interchangeably. Sometimes a sword will be referred to as a blade, particularly in fictional writing (Lord of the Rings et al). You may also see a knife referred to as a blade - he pulled a blade on me. Technically, I guess, the sword is the whole thing and the blade is the sharp metal part, everything above the hilt. However, a sword can be a blade, but a blade is not always a sword. A blade is a metal object with a sharp edge. You can have a razorblade, lawnmower blade, knife blade, a sharpened bit of metal you found on the floor blade, etc.

They're about the same. It'd be better to go online and look at the difference between pictures of blades and pictures of swords, then you can see very clearly how they differ.