It would be visible, or rather, its effects would be.
It might not be as noticeable as, say, a comet in the night sky, but an approaching black hole would resemble a moving distortion in the background behind it due to gravitational lensing.

And the moment it moves through anything with signifcant mass (e.g. a gas cloud), it'd show up as that gas gets pulled into an accretion disk and starts emitting light when the gravitational shear forces superheat it.