嘿 is more casual and playful than 喂 (wèi, hello on the phone) or 嗨 (hāi, hi). It often carries a tone of pleasant surprise or smugness. In texting, 嘿嘿 is a cheeky/mischievous giggle — like a naughty smile emoji.
Mouth radical on the left — also 嘿's indexing radical (Kangxi #30). Marks 嘿 as a vocal-noise interjection: 'hey!', a sharp call to grab attention. Same radical in the entire onomatopoeia family: 哈, 哎, 喂, 嗨, 嘛 — every interjection sits here.
Right side 黑 supplies the sound directly: hēi → hēi with no shift. No semantic contribution from 'black' — pure sound borrowing for the interjection. Same phonetic in 默 (mò), 墨 (mò); these have moved further from the hēi reading but originated in the same cluster.
