A sidenote here : there is no such thing as chroma or luma or colour noise. It’s just noise. How it looks, once it enters our perceptual system, doesn’t change what it is : garbled data on RGB channels.
Similarly, the denoise profile module in darktable doesn’t know any colour or luminance since it operates in camera RGB. That camera RGB has nothing to do with our perceptual system, and will need to meet a colour profile to be remapped to XYZ and then to well-behaved RGB spaces, where chrominance and luminance have a meaning.
Blending the denoise profile in lightness or colour is therefore a theoritical mistake since these are Lab modes. HSV lightness or colour are equally wrong since the conversion RGB → HSV assumes well-behaved RGB spaces, while the camera RGB can be absolutely whatever.
The last changes in denoise profile module (in future darktable 3.0) should make these blending modes irrelevant, since they essentially hide misbehaviours of the denoising algo.