If I couldn't find the port (and believe me, I've lived places where I couldn't buy port, or only could buy real rotgut), I'd use sweet vermouth, sherry, or madiera wine. It wouldn't taste the same, but any of those flavors go well enough with blue cheese and fruit. I'd use one of them before I'd eliminate the booze. Unless your guests are going to be actually comparing your cheese ball to someone elsesm I suspect they'll love it.