How to detect a full black color image in OpenCV Python?

Try this:

# open the file with opencv
image = cv2.imread("image.jpg", 0)
if cv2.countNonZero(image) == 0:
    print "Image is black"
else:
    print "Colored image"

You basically check if all pixel values are 0 (black).

image = cv2.imread("image.jpg", 0)
if cv2.countNonZero(image) == 0:
    print "Image is black"
else:
    print "Colored image"

The snippet above provided by @ebeneditos it's a good ideia but in my tests opencv returns an assertion error when capturing a colored image.

According opencv community, countNonZero() can handle only single channel images. Thus, one simple solution would be to convert the image to grayscale before counting the pixels. Here it is:

gray_version = cv2.cvtColor(image, cv2.COLOR_BGR2GRAY)
if cv2.countNonZero(gray_version) == 0:
    print("Error")
else:
    print("Image is fine")