PNG's imported by Xcode have alpha premultiplied as part of the import phase itself. Have you changed the ccConfig.h file and its pre-multiplied alpha settings? Have you manually set the alpha blending equation?
P.S.: pre-multiplied alpha means that the RGB channels are multiplied by the alpha channel when the image is imported and thus the image exists with the colors pre-multiplied by the alpha value (which is part of what you would do when alpha-blending them).
With pre-multiplied alpha, the normal alhpa-blending equation becomes:
SRC_RGB * 1 + DST_RGB * (1-SRC_ALPHA)
if you mix and match pre-multiplied and non pre-multiplied resources and blending equations you might get artifacts due to pre-multiplying the colors by the alpha channel twice (using the old SRC_RGB * SRC_ALPHA + DST_RGB * (1-SRC_RGB) which was the standard equation in a non pre-multiplied pipeline (and you seem to be falling in this case... with a source alpha value of 0.5 if you pre-multiplied the color value twice you would multiply it by 0.25 darkening excessively), but using it with already pre-multiplied color values) or by using the pre-multiplied blending equation with a non pre-multiplied resource.