use your Aquarium silicone, run a bead of it around the blue plastic seal , keeping it as tight to the lines as poss, on the other part, (or anywhere you DONT want silicone) a light grease with vaseline, so thin you can only just see where youve gone, press the 2 together and leave for awhile till silicone starts to go off, then remove and let it fully cure before trying again. you may have to trim around the edges etc for touchups, but it should give you a molded, near-perfect seal.
:thup: good luck
EDIT: or completely remove teh blue seal and start from scratch with the silicone?? also, may take 2 moulds to get it perfect, just make sure that if you re-do any areas, that there isn't Vaseline. (bit of meths on a cloth should sort that, give you a clean dry spot to go over)
of course, this is all hypothetical and how I would do it, and results may vary...