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Sweet's syndrome, or acute febrile neutrophilic dermatosis, is a rare inflammatory condition that may occur idiopathically or in association with malignancy, drugs, or systemic disease. Among malignancy-associated cases, hematologic neoplasms predominate. Its bullous variant is uncommon and is more frequently linked to underlying hematologic disorders, where it may act as a paraneoplastic manifestation or an early marker of disease progression. We report a 54-year-old woman with long-standing, low-risk essential thrombocythemia, diagnosed in 2011 and maintained on hydroxyurea and aspirin with stable blood counts for over a decade. She presented with an acute onset of painful erythematous swellings progressing to tense bullae over the hands and feet, accompanied by intermittent fever. Histopathological examination of a skin biopsy demonstrated dense neutrophilic infiltration of the dermis without vasculitis, consistent with Sweet's syndrome. Systemic corticosteroid therapy led to rapid clinical improvement, and hydroxyurea was discontinued. Given the known association between Sweet's syndrome and hematologic disease progression, further evaluation was undertaken. Bone marrow examination and cytogenetic analysis revealed a complex karyotype, while next-generation sequencing identified an acquired TP53 mutation along with a CALR mutation, indicating clonal evolution and progression to a high-risk disease state. This case highlights bullous Sweet's syndrome as a rare but important cutaneous marker of clonal evolution in essential thrombocythemia. New-onset neutrophilic dermatoses in patients with previously indolent myeloproliferative neoplasms should prompt dermatologic evaluation and comprehensive hematologic reassessment, as early recognition may facilitate timely diagnosis of disease progression and guide management.