Inverting switching regulator with -80V DC output
Here's a picture of an LT1425 flyback controller producing -9V. To get more voltage you increase turns on output winding of the transformer: -



This is a low power device that is probably suitable for your application but there may be others like LT3574 or LT3573: -



I know it's powered from 6V but it is producing 300V at 5mA and the positive output pin can be grounded. It has a minimum input supply voltage of 3V so should be capable of what you need with the appropriate turns on the secondary.

If you choose a design that has a flyback transformer it is inherently isolating input from output and you can choose to ground the positive output connection thus creating a negative output voltage.