For some reason in minecraft 1.7.2 when I logged on every block had a blue and purple outline diagonal to the block. It really sucks! I can't get a screen shot but if someone tells me how to post it then I will somehow.
Does this happen every time you log on to Minecraft now?
If you haven't already, start with a quick reboot of the computer and fire up the game again. Could be something wonky with the video driver. I've seen similar behavior from Counter Strike: Source every so often, but Alt-Tabbing out of the game and then going back in usually solves it. When it doesn't, I reboot and it definitely solves it. Hopefully your problem requires just as easy of a fix.