Can I use both top and bottom layers for ground planes on BL600?

Yes, you can have GND on bottom (or inner layer if the Host PCB is more than a 2 layer board) and top but please be aware that you must connect the two GNDs together by via (regularly placed). Also, solid (meaning less cuts in GND plane due vias going from top to bottom thus cutting area of the GND plane) continuous GND plane is good for EMC as return currents have smaller loop area (lower inductance).