Sounds like you've got some polygonal errors. When it happens, open the Console by pressing the tilde key (~). In the box that appears, type 'flushtexturecache();' exactly how I just said it, but without the quotes.
If this does not work, try downloading the file
here and replacing your current Blockland.exe with this downloaded Blockland.exe.
Blockland.exe can be found in C:\Program Files\Blockland or C:\Blockland depending on your operating system.