WebGL example

I'm trying to get a simple effect to display using WebGL. Naturally, this means I'm using the fragment shader language defined in the GLSL ES 1.0 specification.

I haven't shown my Javascript code to initialise the canvas, compile and link shaders because, to be honest, I don't fully understand it yet. What I do know is that it sets up a square and uses the fragment and vertex shaders to determine the pixel colors. The following code will just display a white square.

gl_FragColor = vec4(0.0, 0.0, 0.0, 0.0);

However, if I change the alpha component to 1.0 then it will show a black square instead.

gl_FragColor = vec4(0.0, 0.0, 0.0, 1.0); // displays a black square

I'm assuming that the color that is output by the fragment shader must be combined with some previous color. How do I make sure that only the last color (regardless of its alpha value) is what is actually chosen as the color to be displayed?

Or perhaps I've got the order the wrong way around. Perhaps there is a later phase that is combining with the color from the fragment shader to produce the white color. In any case, I know that some kind of blending is going on because when I change the alpha value to 0.5 I get a grey square. I just want to know, where is the white color coming from? And how do I get rid of it?