Post henna mehndi design blister rashes are typically associated with the use of "black henna," which often contains added chemicals like para-phenylenediamine (PPD). Symptoms usually appear 3-10 days after application and can include redness, intense itching, and fluid-filled blisters. 

Managing this reaction involves avoiding scratching to prevent infection and potential scarring. Opting for natural, pure henna and requesting a patch test before full application can help avoid such reactions after applying henna mehndi design. It's important to be cautious of extremely dark henna or promises of long-lasting results, as these often indicate the presence of harmful additives. 

Individuals with a history of skin sensitivities should consider avoiding application of henna mehndi design altogether or consult a dermatologist before use. In severe cases, medical attention may be necessary.