Diamine Registrar's ink is an archival ink and is used by Registrars and the Clergy in the United Kingdom for signing official documents. It is permanent/waterproof. Registrar's ink is an iron gall based fountain pen ink; therefore, it is recommended that you flush your pen with cold water prior to refilling or before storing away. This ink when first applied looks blue and as it oxidizes it turns blue/black. A bottle of Registrar's contains 30 mL of ink.
Please be aware that colour representation cannot be truly accurate owing to the reproduction on different computer systems, variations in scanning and variations in pen, paper and ink characteristics.