SDL - transparency?

I've tried setting up a color key, but it doesn't seem to work. I'm not using sprites of any kind, just drawing rectangles to a surface. Here's what I have so far:

Set-up code

Screen::Screen(unsigned int _width, unsigned int _height) :	width(_width), height(_height), bgColor(0, 0, 0){	// "screen" is of type SDL_Surface*	screen = SDL_SetVideoMode(width, height, 0, SDL_ANYFORMAT | SDL_SWSURFACE);	if(screen == NULL)		logger.Fatal(boost::format("Error creating screen: %1%") % SDL_GetError());	if(SDL_SetColorKey(screen, SDL_SRCCOLORKEY, transparent.GetRGBMap(screen)) != 0)		logger.Fatal("Error setting color key");}

The constructor for the Screen object initializes the SDL_Surface pointer, and attempts to make transparency work, logging any errors in the process.

Drawing code:

void WorldObject::Draw(Screen& target) const{	SDL_Surface* surface = target.GetSurface();	SDL_Rect rect;	rect.w = bounds.max.x - bounds.min.x;	rect.h = bounds.max.y - bounds.min.y;	rect.x = bounds.min.x;	rect.y = bounds.min.y;	if(SDL_FillRect(surface, ▭, color.GetRGBMap(surface)) != 0)	{		string error = "Error drawing to screen: ";		error += SDL_GetError();		logger.Fatal(error.c_str());	}}

It gets the SDL_Surface from the Screen object, then creates a rectangle to draw, and draws it (logging an error if things don't work).

Any ideas?

Thanks/