Final answer:
To a distant observer, a flashing red light falling into a black hole would appear to slow down due to gravitational time dilation, redshift into longer wavelengths, and ultimately seem to freeze at the event horizon due to the intense gravity, becoming nearly impossible to detect.
Step-by-step explanation:
As a flashing red light falls into a black hole, an observer at a safe distance would notice significant changes due to the intense gravitational field of the black hole. Initially, the light would appear to speed up as it approaches the black hole, but as it gets nearer to the event horizon, gravitational time dilation would cause the light to redshift dramatically, changing its frequency to longer wavelengths.
The red light's blinking would seem to slow down, with the intervals between flashes becoming progressively longer. Eventually, the light would become so redshifted that it could move out of the visible spectrum entirely, becoming infrared or even radio waves, before it appears to freeze at the event horizon. At this point, it would take an infinite amount of time for the light to appear to cross the event horizon, making it virtually undetectable due to the tremendous redshift.
Therefore, to an outside observer, the flashing red light would appear to slow down, its light stretching to longer wavelengths, then essentially fade away as it nears the event horizon, a phenomenon reinforced by the fact that any matter nearing the event horizon accelerates to extremely high speeds and emits high-energy radiation like X-rays.