@blackbird, the map in blue is the places you havn't been. You can get a "full" map by finding the map room for the particular section of the game you are in.
Have you played Metroid Prime, its the same thing there.
Once you visit a room, the map turns purple. The single dots in the rooms on the map indicate a special item, i.e. energy tank, bomb, ball, ect.
Best thing to remember in this game is bomb everything you can. That is the easiest way to find hidden passages until you acquire the x-ray scanner.
Remember:
blue doors = any shot
red doors = 5 missiles
green doors = super missile
yellow doors = super bomb
grey doors = you must complete some objective for door to become active
I don't want to give it away to you, but if you absolutely can't figure out where to go, just Google 'Super Metroid Map'. There are plenty of full maps out there. I find it much more gratifying not using these existing maps as they also show you where items are and the hidden areas not automatically shown once you acquire the map for a certain section.