Blå is used with common-gender nouns. En blå hat, en blå kat, en blå stol, etc.
Blåt is used with neuter-gender nouns. Et blåt dyr, et blåt hus, et blåt glas, etc.
Blå is also used with definites and possessives, like den blå blyant and dit blå bord (note that this is regardless of gender); usually it would be the "plural form" of the adjective but there is no plural form of blå.
Blå is used here because of the possessive (din).