What is a painless bluish lump filled with fluid on the lips most likely to be?

A painless bluish lump filled with fluid on the lips is most consistent with a mucocele. Mucoceles are fluid-filled lesions that typically occur when a salivary gland duct becomes obstructed or damaged, leading to a collection of saliva in the surrounding tissue. They can be caused by trauma, such as biting the lip, which explains their occurrence in the oral cavity, particularly on the lips.

Mucoceles often appear bluish due to the translucent nature of the fluids they contain. They are usually painless, which aligns with the characteristics described in the question. In contrast, smoker's keratosis presents as white patches on mucous membranes and is associated with nicotine use, squamous cell carcinoma is a malignant tumor that usually presents with ulceration or other atypical features rather than a fluid-filled lump, and fibromas are benign tumors that are generally firm, rather than fluid-filled, and not typically bluish in appearance.