The Andalusian Blue Gene is one of the most common genes seen in poultry genetics. It is also my favourite gene to play with when trying to create new colours.

The blue gene (Bl) is a dilution of the solid base colour Black. With just one copy of (Bl) it can dilute a black bird to one that is a charcoal/slate grey colour known as blue. Blue comes in varying shades, but the standard is an even dark slate blue on the bird with the chest having nice crisp black lacing. 

Two copies of (Bl) gives a Splash coloured bird. This colour often has a light grey to almost white base with flecks of blue throughout (sometimes with different shades of blue (dark and light) in the flecks. An absolutely stunning bird by far, but not a recognized colour in most breeds. 

This Black, Blue, and Splash birds are often kept together in a pen/flock to add a contrast of colour and because they can be bred together easily by following this chart.