what is the difference between the blue screen key, and the color key (with selection of the blue color)
Not much; they're both pretty close to worthless. Use Ultra Key instead.
Blue Screen and Color Key may work o.k with a graphics type image... but as Colin says...use Ulra Key. Its a surprisingly good keyer.
I will echo what Craig and Colin said. Some effects remain in the application mostly to provide backward compatibility; that's the nicest thing that I can say about the Blue Screen Key effect.
Use the Ultra Key effect.