Why isn't this object becoming transparent in the render? I'm new to Blender and trying to make an image. I'm almost there but something is confusing me. Right now, I have this blue rectangle that I want in the same plane as the other objects. For example, here it is from a couple angles:




and rendered:



Now, I want that blue rectangle to be mostly transparent, so you can see the rest of the shape that it coincides with very clearly. So, I went to the materials tab with that object selected, clicked the transparency box, and messed with the alpha parameter, making it very low:



Now, when I render it, it does look different...but isn't actually transparent:



what am I missing here? How can I achieve this?
You have set your transparency to Mask. To quote the Blender Manual:


  This option simply masks the Background. It uses the alpha channel to
  mix the color of each pixel on the active object plane with the color
  of the corresponding background pixel, according to the alpha channel
  of the pixel. Thus for alpha = 1, the object color is seen – the
  object is completely opaque; but if alpha = 0, only the background is
  seen – the object is transparent (but note that any other object
  behind the active object disappears).


Essentially, any object behind a 'Mask' object will not be visible.
Instead, set the transparency type to Z Transparency or Raytrace: