This is a document that may be carried by ship; it indicates that the shipowner has entered into an agreement with the ITF (the International Transport Workers’ Federation) that the crew are being paid wages and salaries, which are specified by the ITF.
A clause in a charterparty, especially in a time charter, may require the shipowner to guarantee that he will enter into such an agreement and therefore that the ship will carry such a certificate. Failure to carry such a certificate may cause a ship, which may be registered in an “open registry” country, such as Panama or Liberia, to be delayed in certain ports where shipping-related unions are affiliated to the ITF.
Examples are some Australian and European ports. The purpose of the ITF action is to attempt to ensure that shipowners do not take advantage of seafarers from so-called “cheap-labour” countries and pay them very low wages. Charterers are reluctant to take the risk of delay to the ship so they insist that shipowners obtain a “blue certificate”. Incidentally, the term has very little to do with the colour of the document in modern times.