Varicella-Zoster Virus (VZV) Reactivation: Given the patient's history of multiple sclerosis, an immunocompromised state could predispose to VZV reactivation, leading to herpes zoster (shingles), characterized by painful blisters. Low lymphocytes could be a result of the immunocompromised state or the viral infection itself.

Herpes Simplex Virus (HSV) Infection: HSV can cause painful blisters and might be more common in immunocompromised patients, such as those with multiple sclerosis. Low lymphocyte counts could be seen in the context of a viral infection.
Cytomegalovirus (CMV) Infection: CMV is another virus that can reactivate in immunocompromised individuals, potentially causing a range of symptoms including low lymphocyte counts and possibly blister-like lesions, although this is less common.

Disseminated Varicella-Zoster Virus: Although VZV reactivation is considered in the likely diagnoses, its dissemination is a critical condition not to miss, as it can lead to severe complications, including death, especially in immunocompromised patients.
Pneumocystis jirovecii Pneumonia (PCP): While PCP does not typically cause painful blisters, it is a serious opportunistic infection in immunocompromised patients that can present with non-specific symptoms. The low lymphocyte count could indicate a compromised immune system, making this a critical diagnosis not to miss.

Autoimmune Bullous Diseases (e.g., Pemphigus, Pemphigoid): These are rare autoimmune conditions that can cause blistering skin lesions. While they are less likely, they could potentially occur in a patient with an autoimmune background like multiple sclerosis, especially if there's an overlap with other autoimmune phenomena.
Drug-Induced Lymphopenia and Skin Reactions: Certain medications used in the treatment of multiple sclerosis could potentially cause lymphopenia (low lymphocyte count) and skin reactions, including blistering. This would be a rare but important consideration, especially if the patient has recently started new medications.