Contact Dermatitis (Poison Ivy, Oak, or Sumac): This is the most likely diagnosis given the patient's history of walking in tall grass, which increases the risk of exposure to urushiol oil from these plants. The oil causes an allergic reaction, leading to blistering and rash.

Insect Bites (e.g., Chiggers, Mosquitoes, or Fleas): Walking in tall grass exposes individuals to various insects that can cause bites, which may blister. The reaction can vary from mild to severe, depending on the individual's sensitivity and the number of bites.
Cutaneous Larva Migrans: This condition, caused by the larvae of certain nematodes (hookworms), can lead to blistering skin lesions after walking barefoot or in contact with contaminated soil in areas where these parasites are common.

Snake Bites: Although less common, snake bites can occur in tall grass and can be life-threatening, especially if the snake is venomous. Blisters can be part of the clinical presentation, along with other systemic symptoms.
Tick-Borne Illnesses (e.g., Lyme Disease, Southern Tick-Associated Rash Illness): Ticks are often found in tall grass and can transmit several diseases. While the initial presentation might not always include blisters, some tick-borne illnesses can cause rash or blister-like lesions.

Leptospirosis: This bacterial infection can be contracted through contact with water or soil contaminated with the urine of infected animals. It can cause a wide range of symptoms, including skin lesions or rashes, though blistering is less common.
Erysipelas: A type of bacterial infection of the upper layer of the skin that can cause blistering lesions, though it's more commonly associated with other symptoms like fever and well-demarcated erythema. It's less directly related to walking in tall grass but could be considered in the differential if other risk factors are present.