Excellent hourly rail service to Brussels (Bruxelles) from Blankenberge, its the same train as many of your fellow-passengers will be taking as far as Bruges. Do double-check, but I think it leaves at 11mins past the hour.
From Zeebrugge to the rail station at Blankenberge is about 2 to 3 miles. Probably your ship will be running a transfer to the station, on some cruise lines such as P&O its free but it'll only be a few dollars if its charged.
That's your simplest transfer.
Or there'll be taxis at the quayside, but they'll be looking for €50 fares to Bruges & will probably decine the little hop to Blankenberge.
Or the tram, the "kusttram" runs right past the port gate. No tramstop at that point, but it'll be a short walk in one direction or the other. You need to buy a ticket before you board, there'll be a kiosk or a local shop by the tramstop. There's a tramstop right outside the train station in Blankenberge.
For your return train from Brussels, doublecheck if the train splits when it gets to Bruges & make sure you're on the right end of the train for Blankenberge.
Easier than I've made it seem.