Синий is a bit more default than голубой. There is a distinction. However, голубой is usually considered a variation on a more broad area of colours ranging from sky blue to navy blue.
For example, I could describe the Duolingo's header (#24a8e8) as either синий or голубой. When you hover your cursor over a menu, a darker area around the button appears. This colour, I would only call синий. I also would not, probably, use голубой to desribe the shade of blue they use on Facebook (#3b5998)
When a light blue colour has a fair amount or green in it, we tend to use голубой more. There is also a tendency to associate голубой with such qualities as "transparent", "light", "tender" "weightless" and синий with "tangible", "heavy", "material" so the sea is usually синее and the sky is голубое (you could encounter the opposite, with appropriate connotations).