Plain blue
Clear, legible and universally accepted for official documents. Parker Quink Washable Blue is the classic: a medium-bright blue that reads cleanly at any size and is easily photocopied. It is also water-soluble, which makes flushing the pen easy and means it will lift in a rainy pocket - useful to know.
Blue-black
The writer's colour. Blue-black starts blue-black, then as it oxidises and dries it shifts slightly towards a blue-grey-green that shows ink shading beautifully. It reads as professional, it looks alive on good paper, and it is the colour that makes people ask which pen you are using. Parker Quink Blue-Black is the benchmark: reliable on any paper, not too wet, archival when dry.
Black
Clean, definitive, and the right choice for anything that will be scanned or where maximum contrast matters. Black ink is often pigment-based and more permanent than blue. Some black inks (not Quink) can be hard to clean from a pen if left to dry. If you use black ink, flush the pen more frequently.
The recommendation
Blue-black, almost always. It reads as professional but looks better than plain blue; it shows the character of the pen; and the subtle colour shift as it dries is part of what makes writing with a fountain pen satisfying. For a first bottle: Parker Quink Blue-Black.