These sons were Dhritarashtra, who was blind and Pandu, the pale one. It was considered desirable to have another son -who should be \’without any defect. This time the widow sent her maid-servant in her clothes and Vidura •was bom. That is the reason for using the epithet Dasi-Suta for Vidura. See : Vyasa , Vidura .
References :
1. Kohli, Surindar Singh (ed), Dictionary of Mythological References in Guru Granth Sahib, 1993