The definition of Blindness is total but permanent (of six months or more is not stated - what length of time constitutes total). There will be a minimum qualifying period. It just fails to say. A claim may be lodged but then find out about minimum period. The declared blindness may be a symptom of undiagnosed MS but later this will be used as a symptom to demonstrate a pre-existing condition. So disqualifying a later MS claim after it has been diagnosed - actually even before it has been diagnosed.
MS will compromise a possibility of Blindness if the MS fails. It may be total and irreversible in the rarest (perfect!) of cases but it will have been declared in conjunction with the MS claim so will be a pre-existing condition. Claim thrown out. Total blindness but case rejected. Amazing, cynical and diabolical. But look onwards to Terminal illness for some real blood money. Forced to keep paying the premiums by obfuscation for a year - frustrate the claim. Got to pay out on death anyway - minimise loss by collecting as much as possible by premiums.
When is the Blindness condition different from PTD as above? These PTD and separate conditions needs more examination. What's it all mean? Make note of trachoma - not much chance of that. Cataract? Is this correctable by surgery and if so would it not constitute total and irreversible blindness. Could not claim and maybe could not fund an operation to regain sight. Caught in the middle but nevertheless not payable, I think.