Your code will go back to Main after first blink, and then wait again for a command as it see Serin, which wait forever until you send him data.

look into adding a background timer if you want the led to blink, or use a timeout so the Serin will end if no new char is received.

also, some indent in code help to see more clearly

Code:
main:
serin serial,T2400,[1],b0

for b1=0 to b0
  high led
  pause 200
  low led
  pause 200
  next b1

pause 2000
goto main