Contact Dermatitis: This is the most likely diagnosis given the history of hiking, which increases the risk of exposure to allergens or irritants such as plants (e.g., poison ivy, oak, or sumac), insects, or other environmental factors that can cause a blistering rash.

Insect Bites or Stings: Various insects encountered during hiking can cause blistering rashes, including certain types of spiders, mosquitoes, or flies.
Phototoxic Reaction: Prolonged sun exposure during hiking, especially in combination with certain medications (like doxycycline) or plants (like giant hogweed), can lead to a blistering rash.
Allergic Reaction to Insect Repellents or Sunscreens: Some ingredients in these products can cause allergic contact dermatitis, manifesting as a blistering rash.

Stevens-Johnson Syndrome (SJS) or Toxic Epidermal Necrolysis (TEN): Although rare, these conditions are life-threatening and can be triggered by medications or infections. They present with widespread blistering of the skin and mucous membranes.
Erythema Multiforme: This is an immune-mediated disorder that can be triggered by infections or medications, characterized by target lesions and sometimes blistering, which can be severe.

Bullous Pemphigoid: An autoimmune disease that causes blistering of the skin, more common in older adults but less likely to be directly related to hiking.
Dermatitis Herpetiformis: An autoimmune blistering disorder associated with celiac disease, characterized by intensely itchy blisters, which is not directly related to outdoor activities like hiking.
Porphyria Cutanea Tarda: A disorder that can cause blistering of the skin upon sun exposure, due to a deficiency in the enzyme responsible for heme synthesis. It's rare and might not be the first consideration without other specific symptoms or signs.