The phrase "a major blow when" is correct and usable in written English.
It can be used to describe a significant setback or disappointment that occurs at a specific time or in relation to a particular event.
Example: "The company faced a major blow when their leading product was recalled due to safety concerns."
Alternatives: "a significant setback when" or "a serious disappointment when".
Porto's hopes of reaching the Champions League knockout stages were dealt a major blow when they were held at home by Austria Vienna.
The four-time champions' hopes were then dealt a major blow when Vorsah was needlessly booked for the second time when he kicked out at Samba Diakité.
Philip A. Falcone, for one, didn't betray even a hint of the storm raging over LightSquared, the start-up wireless company that was dealt a major blow when the government decided to block its effort to build land-based satellites.
In 1983, the New Zealand anthropologist Derek Freeman delivered a major blow when he published "Margaret Mead and Samoa," charging that Mead had been duped by informants in her pioneering ethnography, "Coming of Age in Samoa".
Colin Powell, George Bush's former secretary of state, today dealt his own party a major blow when he threw his weight behind the bid by Barack Obama to become the next president of the United States.
Colin Powell, George Bush's former secretary of state, yesterday dealt his own party a major blow when he threw his weight behind Barack Obama's bid to become the next president of the United States.
Then in 1995, Franklin Avenue suffered another major blow when Bloomingdale's moved to Roosevelt Field and was replaced by a Sears, Roebuck & Co.
This conflict-affected region suffered another major blow when the monsoon floods affected 75 per cent of the district's 1.7 million people.
Premier suffered a major blow last summer when Trinity Health of Novi, Mich., whose chief executive, Judith C. Pelham, had been on Premier's board, decided to stop using the buying group.
The campaign, Freedom Flotilla II, was dealt a major blow recently when IHH, the Turkish group that led last year's convoy with the ship Mavi Marmara, pulled out, apparently at the bidding of Ankara.
A campaign to close the Indian Point nuclear power plant over fears of a catastrophic terrorist attack suffered a major blow yesterday when the federal government endorsed emergency evacuation plans for it, asserting that the public would be safe.
