Mark 7:15-23, as in Matthew 15:11, Jesus was not referring to prohibited meat either. If you look at the verse in full context starting with verse 1 of both Mark and Matthew, you will see the Pharasees are asking: "Why do your disciples break away from the tradition of the elders? They eat without washing their hands." The response you quoted is not to prohibited meat at all....it's a response to washing unclean hands. Jesus, pbuh, then responds to their hypocracy saying: "Why do you break away from the commandments of God for the sake of your tradition...you have made God's word ineffective by means of your tradition." He was telling them that not observing ritualistic Jewish traditions was not breaking God's Law. If you continue to read on you realize Jesus, pbuh, is saying that if a piece of dirt or dust is accidently eaten because of unclean hands it's not an issue. What matters are the words that come out of your mouth as it is an indication of what is in your heart.
Peace,
Hana