This is often a minor problem, ie. just the [Control Head] needs cleaning. If cleaning the control head cures it, you need to check the quality of the tapes which you are using, as it shouldn't be getting dirty that quickly.
If it was an older machine with several years wear, it could be due to a worn cylinder causing incorrect tension on the tape.
The [control head] is the static head, fitted to the right side of the cylinder head drum. You can clean it with standard tape head cleaning fluid. The video heads themselves, probably, don't require cleaning.
(Blue screen is automatically displayed if the VCR cannot synchronize the picture (rather than just show a noise on screen).
Cobweb, January 2005