Contact Dermatitis: This is the most likely diagnosis given the localized nature of the blisters to the finger area, suggesting exposure to an irritant or allergen. The size of the affected area (2 cm by 1 cm) is consistent with a localized reaction.

Burn (Thermal or Chemical): A small, localized burn could result in a blister of this size, especially if it's a second-degree burn. The fact that it's confined to the finger suggests a minor, accidental exposure.
Friction Blister: Although less common on the back of the fingers, friction blisters can occur anywhere due to repetitive friction or pressure, especially if the individual has been engaging in an unusual activity.
Insect Bite or Sting: Some insect bites or stings can cause localized blistering, especially if there's an allergic reaction involved.

Herpes Simplex Virus (HSV) Infection: Although less common on the fingers, HSV can cause blistering lesions. Given the potential for serious complications, especially if it's a primary infection, this diagnosis should not be missed.
Staphylococcal Scalded Skin Syndrome (SSSS): This condition, caused by staphylococcal toxins, can lead to blistering skin lesions. It's more common in children but can occur in adults, especially those with compromised immune systems.

Bullous Pemphigoid: An autoimmune disease characterized by blistering skin lesions. It's less likely given the localized nature of the blisters but should be considered if other diagnoses are ruled out.
Porphyria Cutanea Tarda: A disorder that can cause blistering on sun-exposed areas, including the hands. It's rare and usually associated with other systemic symptoms.
Erythema Multiforme: A skin condition characterized by target lesions, which can sometimes blister. It's often associated with infections or medication use.