blå is used when it's common gender. You can identify the common gender of a noun by its article "en": en bil, en bror, en fugl. When you want to say that the bird is blue, you say "en blå fugl".
blåt is used when the noun is not the common gender, but a singular neuter. You can indicate that from the article "et": et dyr, et jordbær. When you want to say that the animal is blue, you say "et blåt dyr".
Now we have "en blå fugl" and "et blåt dyr". These are indefinite and singular forms.
There are also plural and definite forms. Definite means, it is not "a bird" but "the bird". For the plural and definite forms you use "blå". E.g. "den blå bil" (the blue car).