All PICs have protection diodes on the pins which prevent the pins going outside the range Vdd+0.3V and Vss-0.3V.

Because your feed is via a high impedance resistor you wont damage the PIC but when you are expecting the output to be -15V it will only be -0.3V

I am guessing that because you have -15V in your circuit you have dual supply rails so could you not use an Opamp to invert the pic output to be in the range 0V to -5V. You would then have -ve voltage to drive your PIC.

Another possibility is to use a hardware PWM in the PIC to generate a negative voltage. Search for "negative voltage" and "LCD".

Hope that helps