[Python] Python: perform blur only within a mask of image

I have a greyscale image and a binary mask of an ROI in that image. I would like to perform a blur operation on the greyscale image but only within the confines of the mask. Right now I'm blurring the whole image and than just removing items outside the mask, but I don't want pixels outside of the mask affecting my ROI. Is there a way to do this without building a custom blur function?

hoping for something like:

import scipy
blurredImage = scipy.ndimage.filters.gaussian_filter(img, sigma = 3, weight = myMask)


@stefan:

blur = 3
invmask = np.logical_not(mask).astype(int)

masked = img * mask
remaining = img * invmask

blurred = scipy.ndimage.filters.gaussian_filter(masked, sigma = blur)
blurred = blurred+remaining


Dilate approach:

blur = 3
invmask = np.logical_not(mask).astype(int)
masked = img * mask
masked2 = scipy.ndimage.morphology.grey_dilation(masked,size=(5,5))
masked2 = masked2 *invmask
masked2 = masked + masked2
blurred = scipy.ndimage.filters.gaussian_filter(masked2, sigma = blur)

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