Why do some of my blacks look blue?

[pokefan]

At times some of the blacks on my display turn blue. It's not all the time and isn't even consistent through out the same movie. Last night I was watching Australia and the scene where Lady Ashley and Drover start to kiss behind the tree, the black area between them as they meet turned blue. But blacks throughout the rest of the movie seem fine. I've seen it in other movies as well but can't remember off the top of my head when. I'm pretty sure I've seen it in parts of TDK as well. But yet I can throw in a calibration disk and the blackest blacks on there look fine.

Is this a encoding issue? Maybe a transfer? I just don't understand why it does it at times but it's not consistent with other black levels?