I have been told that this is kind of a common problem with transparent textures, but didn't get any further information on how to solve it.
In my code, I'm trying to render this texture (a square with rounded corners, where rounded corners have some alpha)
What I get instead is this
Notice those greyish places on the corners of the textures - where rounded corners are supposed to be.
What could be causing this? I have a pure-white texture, so I don't expect a single pixel to get any darker than the background. All pixels should have at least the color of background, but as you can see, there is something darker.
Any help would be highly appreciated.