Is this closer to what you had in mind? It was done in PL 3.2 using the local adjustment brush with 100% feathering. I painted over the flower and some of the area around it, and then completely desaturated the mask before inverting it. I then added more micro contrast to the flower and slightly modified the color using the color wheel.
You can see the gradual transition of the leaves, as well as the petals near the upper edge of the image, from color to monochrome. I’ve also included the original edited version I used for this example.
With feathered and desaturated mask after inversion.
Original edited version.