it could be the exhaust valve oil seal wants replacing! blue smoke means oil is comming down the valve stem and going into the exhaust: black smoke is where oil is beening burnt in the cylinders i
valve stem seals are a common problem on 205 gti's. most likely that. remember you can replace stem seals without removing the head by feeding string into the bore throigh the spark plug hole, turning the engine (BY HAND!!!) to comress the string against the valve and then removing the valve spring as normal. saves loads of time and countless hundred pounds! just remember to keep the end of the string dangling out the plug hole otherwise its a real sod to get out!!!