Why are there two blue mark periods in the 1990s?

Belleek’s mark guide lists an 8th mark (1993–1997) in blue, followed by a 9th mark (1997–2000), also blue. The reason given is technical, as the first blue mark had firing problems where parts of the mark could burn off, leading to extra refiring and cost. A new artwork style was adopted for packaging around that time, and Belleek used that moment to update the stamp design into a second blue mark. If you collect 1990s Belleek, these two blues can look close at a glance, so use the full stamp artwork, not colour alone, to place it correctly.