Is it a good sign if a patient in coma's blood pressure increases when you talk to them?

Difficult . BP change on talking to the comatose patient means his autonomic system is responding to some stimuli not necessarily speaking. I am not aware of study that has answered to this parameter for prognosis.
Maybe. Any physiologic response to an external stimulus can be a sign of responsiveness, which is better than unresponsiveness. So, if a coma patient has any change when stimulated (heart rate, blood pressure, movement), it can be a sign they sense the stimulus. We look for purposeful movement (pulling away from pain, grimacing) and eye opening as signs a patient may be starting to wake up.