Go into the Blockland folder Base/Data/Shapes, then there's a bunch of textures in there:
black25.png
black50.png
black75.png
black.png
blank.png
blue.png
darkRed.png
gray25.png
gray50.png
gray75.png
green.png
lightBlue.png
lightYellow.png
palegreen.png
red.png
whiteCheck.png
yellow.png