A tautology is not an argument, but rather a logical proposition. A logical argument may contain tautologies.
To be a valid logical argument (using the traditional rules of predicate logic), not only do all of your statements need to be true, but the argument needs to prove the statement being argued.
If you are making the argument that "the sky is blue if the sky is blue," and your proof is a single logical statement: "the sky is blue if the sky is blue," provided as a tautology, then you have indeed made a valid argument. However, typically such is not the sort of argument that people make. Typically the argument would be a more interesting statement such as "the sky is blue." As part of such an argument, one might use the tautology above. That tautology would be valid, but on its own it would not prove the intended statement "the sky is blue," so the whole argument would not be valid.