That is a fail in the textures.
To get rid of it, go search 'squiddeey' or 'manty' on forums and download their textures.
Then, go to USER/MyDocuments/Blockland/base/data/shapes and replace the existing textures with the new ones.
AND BE SURE TO MAKE THEM READ-ONLY OR THE LAUNCHER WILL OVERRIDE THEM with the default crappy ones.