Methylene Blue or Food Coloring: The most common cause of blue urine is the ingestion of substances that can color the urine, such as methylene blue (used in some medical procedures or as a dye) or certain food colorings. This is because these substances are excreted by the kidneys and can directly color the urine.

Medications: Certain medications, like those containing the dye indigo carmine, can cause urine to appear blue. Other drugs, although less common, might also alter urine color due to their chemical structure or metabolites.
Foods and Beverages: Consuming large amounts of foods or beverages with strong colorants, such as blueberries or certain energy drinks, can potentially change the urine color, although this is less likely to result in a distinctly blue color.

Porphyrias: Although rare, porphyrias are a group of disorders that can lead to the accumulation of porphyrins, which can cause urine to darken or change color. Some types might result in a blue or purple discoloration, especially when exposed to sunlight. Missing this diagnosis could lead to delayed treatment of a potentially serious condition.
Melanuria: In cases of melanoma, especially metastatic melanoma, melanin can be excreted in the urine, potentially causing it to appear darker or even blue. This is a critical diagnosis not to miss due to the implications for cancer treatment.

Alkaptonuria: This is a rare genetic disorder that affects the breakdown of certain amino acids, leading to the accumulation of homogentisic acid. When this acid is oxidized, it can turn urine dark, but in some cases, it might appear blue, especially after standing.
Pseudomonas Infection: Certain infections, like those caused by Pseudomonas bacteria, can produce pigments that might alter urine color, though this is more commonly associated with greenish discoloration. However, in rare instances, it could potentially cause a blue appearance, especially in the context of a urinary tract infection.