תלמוד על מנחות 85:8

If one buys a ṭallith from an idolater or a Samaritan and found blue threads in it, it is invalid;²⁰The colour might not be that of the ḥillazon blue, which is the only colour that is recognized. but [if a ṭallith is bought] from an Israelite, it is presumed to be valid.