I’ve explained the case of Sathyakaama Jaabaala in Implication of Caste for Moksha. Now to add to it, they say only Prathiloma marriages (lower man marrying a higher female) are forbidden, and not Anuloma marriages (higher man marrying a lower female).
But then Devayaani, the daughter of Maharishi Shukraachaarya married the Kshatriya king Yayati, which is a Prathiloma marriage. Yadu was born from that marriage, and it was from Yadu Kula that Krishna was born. (But idiots say that Yayati asked the blessings of Shukraachaarya and had the marriage sanctioned to not incur any Paapa).
Note: British made the caste system worse by forcing everyone to have a surname. That is why Sathyakaama (whose mother’s name was Jaabaala) was considered a Brahmin without much question.
In case of Vidura, he was blessed
Vyaasa was born to Sathyavathi, who was born to an Apsara and hence had no Varna. He was born from Sathyavathi and Maharishi Parashar. The mother is also involved in deciding the Varna if one is to go by the logic of the Smrithi (the same one that makes this claim, making it self-refuting). (Mahabharata, Amshavatarana Parva, Chapter 63, Gitaprem Press)