We are on the same mini cruise as you I think. I rang Fred Olsen to ask the same question and was given the stock reply, if the nearest town is more than ten-fifteen minutes walk away there should be a shuttle provided but that was decided by the ship and that they could give me no more information!
In the event of there not being a shuttle provided I beleive there is a tram that runs from Zeebrugge to Blankenburge every fifteen minutes.
They usually lay on a shuttle bus if it take more than around 15 minutes to walk to the town centre or there are security restrictions in the port. Give Fred. Olsen Cruise Lines a ring (01473 746 160) and pose the question.