BZFlag 2.4.2 introduced the ability to separately control the opacity of the chat panel and the radar. The GUI interface forces both settings to full (1.0) when either of them are set to full, and prevents the other from being full when one is decreased below full. However, when the client is upgraded from a previous version to 2.4.2 with a configuration that specified a full opacity for the panel, this causes the radar to blur. This is because the default value of 0.3 is inserted for the radar opacity, while the panel opacity is at 1.0.
A workaround to the problem is to adjust the radar and panel opacities in the Options > GUI Settings menu so that both are set to full, or that neither is set to full.
A proper fix would be to have the configuration upgrader check this condition, or to do some other checks elsewhere during normal operation of the game.