What might cause a black spot in your direct line of vision that does not go away?

On the information given, the most likely cause is a simple floater. This is a change in the vitreous humor of the eye which casts a shadow onto the retina. If this happens to be in the line of sight, it will move with the eye and always appear directly ahead. Floaters rarely indicate serious problems, and most people acquire a few over time, but a sudden fresh batch of floaters, especially if accompanied with any bright flashing lights in the vision, should be followed up medically for the UNLIKELY (2% max) chance of an early retinal detachment. Other causes of a spot in the direct line of vision are rarer but indicate a problem with the macula. This can be macular degeneration which is relatively common in the elderly, central serous maculopathy which is more common in younger people but is often self-resolving, localised macular haemorrhage and reaction to certain medications and drugs most commonly anti-malarials.