Stasis Dermatitis: This condition is a common cause of red and black rash on the lower legs, particularly in individuals with poor circulation or venous insufficiency. The rash is often accompanied by swelling, itching, and skin thickening.

Eczema (Dyshidrotic or Nummular): Eczema can cause a red and black rash on the lower legs, characterized by small, itchy blisters or patches.
Contact Dermatitis: Irritant or allergic contact dermatitis can cause a red and black rash on the lower legs, especially if the individual has come into contact with an offending substance.
Cellulitis: A bacterial infection of the skin and subcutaneous tissue, cellulitis can cause a red, swollen, and tender rash on the lower legs.

Necrotizing Fasciitis: A life-threatening infection that can cause a red and black rash on the lower legs, accompanied by severe pain, swelling, and crepitus.
Meningococcemia: A severe infection caused by Neisseria meningitidis, which can cause a red and black rash on the lower legs, as well as fever, headache, and neck stiffness.
Rocky Mountain Spotted Fever: A tick-borne illness that can cause a red and black rash on the lower legs, accompanied by fever, headache, and abdominal pain.

Vasculitis (e.g., Henoch-Schönlein Purpura): A group of disorders that cause inflammation of the blood vessels, which can lead to a red and black rash on the lower legs.
Pigmented Purpuric Dermatosis: A rare condition characterized by a red and black rash on the lower legs, caused by inflammation of the small blood vessels.
Kawasaki Disease: A rare condition that can cause a red and black rash on the lower legs, accompanied by fever, lymphadenopathy, and desquamation of the hands and feet.