Sorry but I do not have a firm answer since I program in the text based BASIC programming language.
The PICAXE website makes reference to FOR...NEXT loop structures but the pdf manual does not expand on loop structures though the toolbox image shows loops.
In BASIC there is an optional parameter for FOR...NEXT loops - this is the STEP parameter.
By default the step/increment is +1 but by adding the word STEP a different increment can be defined.
For example:
FOR b0 = 1 to 50 STEP 5
; do something
NEXT
And
FOR b0 = 50 to 1 STEP -5
; do something
NEXT
Hopefully the above may give you a clue to a decrementing loop in BLOCKLY.