Seems like a dup of Uploading Grayscale images causes the image to be darkened. The reason that the image is going black when uploaded is that the source image is so dark to begin with. Testing to be sure:
And with a single de-grayed pixel (top-left pixel changed from RGB(53,53,53) to RGB(54,53,53)):
Edit: Make sure to check the full size images. The thumbnails are reduced in size enough that the color shift doesn’t have an effect on the darkening of the image.