There are options for choosing the size of the blocks and the colour. There is loads of scope to change the program to make a more sophisticated drawing package - enjoy!
UPDATE 2015-05-05: Download the Windows executable version.
REM the blockie graphix
REM Tim Goliath Street
REM 11/10/2012
REM screen parameters
X_WIDTH% = 1920
Y_WIDTH% = 1520
ON ERROR RUN
MODE 12 : OFF
REM set the graphics cursor to somewhere in the middle of the screen
x% = X_WIDTH% DIV 2
y% = Y_WIDTH% DIV 2
REM display instructions
*font Courier New, 40b
PRINT " BLOCKIE GRAPHIX"
*font Courier New, 12
PRINT " By Mr Street"'
*font Courier New, 20b
PRINT " INSTRUCTIONS"
*font Courier New, 12
PRINT " Use the CURSOR KEYS to move."
PRINT" Press 1-9 for different colours"
PRINT" Press SPACE for no colour"
PRINT" Press DELETE to, erm... delete"
PRINT
PRINT" (Press any key)"''
g% = GET
REPEAT
INPUT " Enter block size (5-100) : " diam%
UNTIL diam%>=10 AND diam% <=100
diam% *= 2 : REM even numbers are best!
CLS
REM start with a random colour
currentCol% = RND(8)
REM MAIN LOOP
REPEAT
REM display the cursor
GCOL 7
RECTANGLE x%,y%,diam% , diam%
REM get user input
g% = GET
REM remove the cursor
GCOL 0
RECTANGLE x%,y%,diam% , diam%
REM process user input
CASE g% OF
WHEN 136
REM left
IF x%>diam% THEN
x% -= diam%
ENDIF
WHEN 137
REM right
IF x%<X_WIDTH% THEN
x% += diam%
ENDIF
WHEN 138
REM down
REM left
IF y%>0 THEN
y% -= diam%
ENDIF
WHEN 139
REM up
REM right
IF y%<Y_WIDTH% THEN
y% += diam%
ENDIF
WHEN 135, 8
REM delete, backspace
currentCol% = 0 : REM set to black
WHEN 32
REM space
currentCol% = -1 : REM no colour
WHEN 49,50,51,52,53,54,55,56,57,58
currentCol% = g%-48
ENDCASE
REM draw a rectangle in the current colour
IF currentCol% <> -1 THEN
GCOL currentCol%
RECTANGLE FILL x%, y%, diam% , diam%
ENDIF
UNTIL FALSE