Is it safe to have a ground plane under a Bluetooth module for a DC-DC converter?
The manufacturer of the Bluetooth module will typically specify a keep-out area around the antenna area of the module, which usually will extend at least 1/4" in all directions.

In some cases, even without a ground plane, the presence of G10 or etched PWB material under the antenna section can be sufficient to detune the antenna and affect its performance.
In all cases, it is best to refer to the documentation for the Bluetooth module you are using.

The module you are referring to uses a TI cc2451 chip. Looking at the extensive documentation on this chip on the TI web site, I found Application Note AN043 which describes different antenna designs. When referring to the type of antenna used in your module, they state that " It is also recommended to use the
same thickness and type of PCB material as used in the reference design. [...] To compensate for a thicker/thinner PCB the antenna could be made slightly shorter/longer."

So simply placing the module over an un-etched PWB will be largely similar to increasing the thickness of the PWB, which will affect the antenna's performance. It is possible that the module's antenna was designed to be placed over unetched PWB but without knowing for sure, you are betting either way. 

Good practice is to keep an area at least 1/4" or 1/2" around in all directions free from metal. You most likely will not suffer severe degradation.
A ground plane under the antenna is certainly a bad idea.