As if in defamation, the poison born from the Ocean of Milk manifested its potency by marking Lord Śiva’s neck with a bluish line. That line, however, is now accepted as an ornament of the Lord.
Srimad Bhagavata Purana
श्रीमद्भागवतमहापुराणम्
मूल श्लोकः
(हर हर नमः पार्वतीपतये हर हर महादेव) तस्यापि दर्शयामास स्ववीर्यं जलकल्मषः । यच्चकार गले नीलं तच्च साधोर्विभूषणम्
(hara hara namaḥ pārvatīpataye hara hara mahādeva) tasyāpi darśayāmāsa svavīryaṃ jalakalmaṣaḥ | yaccakāra gale nīlaṃ tacca sādhorvibhūṣaṇam
SB 8.7.43