If you don't use the idea of switching video sources you need to syncronise the blank video source with the real video source. That is the sync pulses must be exactly in phase. This is a difficult thing to do and impossible on an arduino because these pulses must remain in phase even when the signal that syncronised them is gone. This would involve a complex phase locked loop and is not something that could be simply described in a forum post. It would be quite an involved design. Google the term genlock this reffers to a circuit that locks video sources together but requires your video source to be able to accept a syncronising oscillator input.
What form is this video in? Composite or RGBS or som other form. This is important for any solution.