I tried to render a .png with transparency as well. With the basic setup (blending not activated) it didn't render properly.
I never activated blending, but I did add this:
My issue was the alpha was never cleared. Meaning if I rendered with 0.1 alpha, it would take 10 frames until the alpha was "stacked" back up to 1. Adding the call above cleared the alpha, just like you would clear color.
I know this is an old question, so the solutions then were different. enableBlending should still work, but I am not sure if this prevents "stacking" of alpha. Clearing the alpha bits is probably the easiest way to prevent the solid look.
As for the actual rendering, that is the same as always.
//Viewport, camera, logic, etc
//draw your image(s) here