Translation:The bird which is blue
The rules for that and which have changed over time. At present, that and which can both be used to form restrictive clauses (provide information essential to the sentence). The bird that/which is blue is the only bird in the room. You cannot remove the clause without destroying the meaning of the sentence. Commas are not used.
Which is used for non-restrictive clauses and provides a sentence with non-essential information. "The book, which I am reading, is about apples" could have been written as "The book is about apples." Non-restrictive clauses begin with "which" and never "that"... at least until this rule changes. :) Non-restrictive clauses are normally set off by commas.
In practice, people write the way they want to write, but grammar attempts to establish a norm for the times.