The simplest way to repair a cosmetic crack is to inject epoxy-based resin into the crack with a caulking gun. If more than one layer is necessary to fill the crack, wait till the epoxy dries before adding another layer.
If there is water behind the wall, the homeowner is likely to notice an efflorescence in the area of the crack before long. A more extensive repair involves chiseling a upside-down V into the crack and then inject hydraulic cement. This repair is temporary, however, as hydraulic cement doesn't bond well with concrete, and it eventually weakens because of fluctuations in humidity and temperature. Again, the homeowner eventually notices efflorescence and then water leaks around the repair.
Urethane is an alternative to epoxy and hydraulic cement. While it is somewhat flexible and can move when the wall moves, it doesn't expand with the crack. Polyurethane polymer does expand with the crack and bonds to both dry and wet concrete. Repairing a crack with an injection of polyurethane polymer is also minimally invasive and does not take much time.
A horizontal crack in a block wall, especially one that echoes a crack in the floor, may be a sign that the foundation is failing and needs to be inspected by a professional.