Many people erroneously believe that this is a misspelling of petrel blue, and that the term refers to the sea bird. However, the OED gives only this spelling. The term may originally have pertained to petrol-derived compounds such as paraffin, which is often coloured blue.
Most likely the word "petrol", which describes a colour of bluish green, is derived etymologically from the word "petroleum" (crude oil). Petroleum varies greatly in colour, some are colourless, many of them are of nice bluish green colour. Not the petrel or the adding of colours to fuel (British English: petrol) etc. are the reasons, why the colour petrol is called petrol.