People here will have heard of JetBlue after they start advertising the route and eventually launch flights. They aren't a new start or some obscure company. You don't have to visit a huge US airport to see JetBlue aircraft either, anyone who has been to America will have at least a passing awareness of the airline. And anyway all that matters is price. When people search for cheap flights to the usa and jetp blueb popup they will go for it.
Norwegian and WestJe are two similar examples of foreign airlines entering the UK market and doing well without any previous awareness.