it is shunning when you deny them their right to identify as Black. When someone says, "you are not Black you are mixed" they are denying that person the right to thier culture and are disconnecting that person from their own family history. It is an underhanded comment that divides Black people. Black is not merely genetic, it is a social construct that operates today as a signifier of culture and history. If someone identifies as Black, they are identifying with the history and culture of their family and their people. They have every right to identify as such as it is their heritage. They have a right to identify as mixed or as Black. It is their choice how they identify. If some mixed people only identify when it is convenient, that person is shallow, but that person is not all mixed people.
Also, truth is, even on the continent of Africa, there is no single "pure Black race". People from Sudan do not look like people from Zimbabwe, and they dont look like the people from Nigeria, and they dont look like people from Ethiopia. "Black" is a way for people of "color" to unite. When you start saying "mixed people aren't Black", you need to check yourself, because unless you straight outta Sudan, there is someone out there who is Blacker than you. If someone is genetically mixed but identifies with their Black heritage, no-one has the right to deny them that.