Arguably, yes, the message of racism is successfully communicated, and this is achieved through the way in which Walter faces down Mr. Lindner at the end of the play. One of the main ways in which racism as a threat is conveyed is through Lindner's offer to the Younger family to buy their house that Mama has just purchased, at a profit, in order to ensure that his white community does not have to include a black family. Note how he phrases this:
What do you think you are going to gain by moving into a neighbourhood where you just aren't wanted and where some elements--well--people can get awful worked up when they feel that their whole way of life and everything they've ever worked for is threatened.
The reference to "some elements" which is quickly modified highlights the threat that Lindner is refering to. However, it is significant that this play ends with the Younger family rejecting Lindner's offer and choosing to move into this white community, which will no doubt result in significant problems for them when they are there. The playwright means to hold up the Younger family as an example of how blacks should fight racism and stand up for themselves, and Walter's success in doing so, especially when he has just suffered a financial loss himself and could do with the money Lindner is offering, is the way in which Hansberry successfully communicates the need to fight racism.