Note: Black thread being tied so as protection from an evil-eye, black magic etc.
بسم الله الرحمن الرحيم
(1) If the person who ties black thread around ankle, arm, wrist or waist does not consider it effective in itself as well as no other shirkiya or kufriya belief is attached to it, then it may not be called shirk, but it is unlawful to tie it.
(2) If someone offers fardh or nafl salah tying black thread then his salah shall be valid, but makrooh.
(3) In order to avoid evil eyes, black magic etc one should do any lawful act and for the same one should consult any reliable and authentic Amil. It is not correct to contact any non-Muslim or unauthentic Amil.
Allah knows Best!
Darul Uloom Deoband