The Color transfer mode in GIMP is not luminance neutral.  If a noisy image is duplicated into a new layer, the new layer blurred, and the transfer mode of the new layer set to Color, the procedure worsens luminance noise.  There is enhanced 'blotching', and greater contrast of the luminance noise.  This should not happen, all that the procedure should achieve is that the colour component of the noise is eliminated, although of course this will make the colours somewhat more muted and perhaps 'leaking' over colour boundaries.  The problem is also evident if the procedure is reversed, with the bottom layer being blurred and the top layer using the Value transfer mode.  The problem is also the same if the Fade dialog is being used for the transfer instead of a new layer.