I have never used a mac, or a labtop, nevermind both, to play blockland on.
But I can however, entice you to try to reinstall the game. You may have corrupted files / textures / and or addons. If the reinstall doesn't help, delete the newest addons you have. (Since this started) if you can't remember, wipe the addons out and redownload the needed ones, unlike a mech or the present. (Useless USELESS addons...)
However, many mac (and vista) computers have problems with blockland, it could also be an issue with your graphics card, which can be updated. This however, is a poor response. Sorry I can't help more, but something is better than nothing.