Without slaying the enemies in the encounter, Vişņu does never go black, but it is hard to fulfil my desire from Rāvaņa, well protected by the boon.
नाहत्वा समरे शत्रु विष्णुः प्रतिनिवर्तते। दुर्लभश्चैव कामोऽद्य वरगुप्तादि रावणात्॥