Negative voltage from positive output DC/DC
This is a typical POL converter without galvanic separation.

As long as you prevent any further galvanic connections between your 200V circuit to the primary circuitry, you are fine.

After all, polarity is basicly mathematics. 

But you probably will want connections between different parts of your circuitry. Then you will run into trouble with this POL-converter. You can look up, if there is a fully galvanically separated DC/DC converter with your spec, or a POL-converter with negative output.