Śrī brahma-saṁhitā 5.38

The Śyāmasundara form of Kṛṣṇa is His inconceivable simultaneous personal and impersonal self-contradictory form. True devotees see that form in their purified hearts under the influence of devotional trance. The form Śyāma is not the blue color visible in the mundane world but is the transcendental variegated color affording eternal bliss, and is not visible to the mortal eye. On a consideration of the trance of Vyāsadeva as in the śloka, bhakti-yogena manasi etc. [SB 1.7.4], it will be clear that the form of Śrī Kṛṣṇa is the full Personality of Godhead and can only be visible in the heart of a true devotee, which is the only true seat in the state of trance under the influence of devotion. When Kṛṣṇa manifested Himself in Vraja, both the devotees and nondevotees saw Him with this very eye; but only the devotees cherished Him, eternally present in Vraja, as the priceless jewel of their heart. Nowadays also the devotees see Him in Vraja in their hearts, saturated with devotion although they do not see Him with their eyes. The eye of devotion is nothing but the eye of the pure unalloyed spiritual self of the jīva. The form of Kṛṣṇa is visible to that eye in proportion to its purification by the practice of devotion. When the devotion of the neophyte reaches the stage of bhāva-bhakti the pure eye of that devotee is tinged with the salve of love by the grace of Kṛṣṇa, which enables him to see Kṛṣṇa face to face. The phrase “in their hearts” means Kṛṣṇa is visible in proportion as their hearts are purified by the practice of devotion. The sum and substance of this śloka is that the form of Kṛṣṇa, who is Śyāmasundara, Naṭavara (Best Dancer), Muralīdhara (Holder of the Flute) and Tribhaṅga (Triple-bending), is not a mental concoction but is transcendental, and is visible with the eye of the soul of the devotee under trance.