Blue was historically associated with Saint Patrick before green became dominant.
Saint Patrick's blue, a sky-blue shade, was used in heraldic and official contexts connected with Ireland. The color appeared in the Order of Saint Patrick, founded by King George III in 1783, and in some representations of Irish institutions and symbolism.
Green later became the color most strongly connected with Saint Patrick's Day, Irish nationalism and Irish identity. Its prominence grew through the use of green clothing, shamrocks and political symbolism, especially from the eighteenth century onward. The modern holiday therefore looks overwhelmingly green even though earlier official associations could be blue.
Saint Patrick's blue should not be confused with the blue of the modern flag of the Republic of Ireland, which is not part of that national flag. Color traditions surrounding Ireland changed over time and varied by context.