Does an LED TV that is displaying "black" emit enough light to trigger an LDR?
If not, what are some options I could use instead of an LDR?


Monitoring something does not mean there is a mechanism to "make sure it stays 'online' " - that needs some form of interaction with a control but, assuming you mean you just want to detect if it goes off-line then, why not monitor the AC current it takes - if it goes off-line, almost for sure the current taken down its power cable will reduce significantly and this, in the case of an AC power feed, is easy to do with a current transformer on either the live or neutral wire. It could also be done with a hall effect sensor and this would work if the power source is DC.