There seems to be no option to set border alpha. I'd like to turn the border off as well. I tried setting a border size of 0, but that results in severe graphical anomalies. Also, adjusting border size seems to reset the border color to white.
Manually entering -0.5 seems to reduce it to single pixels in the corners which isn't too bad.
Also, is there a standalone addon that displays the names of people in group who ping the minimap? That's about the only other feature I can think of atm that I miss (trying BasicMinimap+Blipstick+BattleObjectsMover as a replacement for Chinchilla)