Black list. Because oil tankers frequently load oil from Islamic countries these countries may place certain ports on a "black list" such as Israeli or South African ports, and if tankers are known to have called at ports in those countries, the tankers' names are on this black list and the ship can be boycotted and refused entry or cargo.
The Arab League used to have very good intelligence service that could determine if a ship was on a black list even if there was a name change. Shipowners could go where the black list was administered, for example in Damascus, and negotiate for the ship to be removed but this was a costly task. In a tanker charterparty a clause-may require the owner to warrant that the vessel is not blacklisted nor boycotted by any country in the Arab League.
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