"Blighty" was first used in India in the 1800's, and meant an English or British visitor. It's thought to have derived from the Urdu word "vilāyatī" which meant foreign. The term then gained popularity during trench warfare in World War One, where "Blighty" was used affectionately to refer to Britain.

Why do we call it blighty?

'Blighty' is another one of those Indian words that made it into English during the days of the Raj. It comes from the Hindustani vilayati, which sounds like 'blighty' in many regional dialects. ... The word really caught on in the First World War.

Where did the word blighty originate?

The word derives from the Bengali word biletī, (older sources mention a regional Hindustani language but the use of b replacing v is found in Bengali and not Urdu) meaning "foreign", which more specifically came to mean "European", and "British; English" during the time of the British Raj.