Powdery Mildew

• Powdery mildew can take on a bluish appearance in a lawn. The fungus that grows on the surface of grass blades actually develops as white powder, but the white coating is light and the green grass beneath shows through, giving the fungus a blue coloring. If you suspect the blue fungus in your lawn is powdery mildew, take steps to eliminate it. Remove any obstacles that block air circulation around the grass and cut away dense foliage that blocks sunlight. If powdery mildew is left untreated, the grass turns yellow and eventually dies.
  
  

Typhula Blight

• Another fungal disease that can cause the growth of bluish mold on grass leaves is Typhula Blight. It is caused by two varieties of fungi that become active in extremely cold temperatures, such as those found on the ground beneath standing snow. Because Typhula Blight develops as a gray fungus, it, like powdery mildew, may appear blue on green grass. In extreme cases, fungicide can be used to stop the spread of the disease, but the disease usually goes away on its own once the ground temperatures become warmer.
  
  

Slime Mold

• Moisture activates the lawn disease commonly known as slime mold, and, according to Jessica Hyatt of North Carolina State University, slime mold can be quick to develop after rain. Slime molds are most prevalent in slow-drying areas that have poor drainage or do not receive adequate sunlight. Slime mold, which can range in color from white to dark blue, poses no danger to grass health and will go away once the grass dries.
  
  

It's Not a Fungus

• A blue tint to grass does not always mean a fungus. If the blue tint appears in the midst of a drought, it may be due to wilting, according to Gail Schumann and Richard Cooper of the University of Massachusetts. If wilting is the cause of the blue fungus-like development on grass, the blue will go away once the grass receives adequate water.