OpenCV's Gaussian smoothing darkens the whole area of the image. This is because the calculation is done using integer values.
import numpy as np
import cv2
image=cv2.imread('image.bmp',cv2.IMREAD_COLOR)
output8ui=image.copy()
output64f=image.astype(np.float64)
for i in range(10000):
if i%1000==0:
print(np.average(output8ui),np.average(output64f))
output8ui=cv2.GaussianBlur(output8ui,(15,15),7)
output64f=cv2.GaussianBlur(output64f,(15,15),7)
cv2.imwrite('gauss8ui.bmp',output8ui)
cv2.imwrite('gauss64f.bmp',output64f)