The Dark Lord
Tirumāl (Tamil: tiru = sacred/auspicious + māl = the dark one / the great one; 'the auspicious dark one' or 'the supremely great sacred one') is one of the most affectionate Tamil names for Bhagavān Viṣṇu. The element māl derives from mā (great/dark) — a beloved descriptor of Viṣṇu's beautiful dark complexion, celebrated throughout the Divya Prabandham.
Color Theology
Bhagavān's dark blue or black complexion (śyāma) is theologically significant: like the dark rain cloud that brings relief to parched earth, His darkness represents the coolness and relief of His grace descending upon suffering souls in saṃsāra. The Āzhvārs rhapsodize endlessly about the Lord's dark beauty — 'kārmugil varṇan' (cloud-dark complexioned), 'nīl varṇan' (blue-dark complexioned).
In the Prabandham
Tirumāl appears throughout the Nālāyira Divya Prabandham as one of the most common epithets. Toṇḍaraḍippoḍi Āzhvār's Tirumālaai (literally 'of Tirumāl') is one of the most beloved prabandhas, celebrating the Lord of Śrī Raṅgam.