What does a blue light by the toilet sign mean?
You don't indicate which aircraft make and model you were flying in. I'm going to answer with some general information regarding Airbus aircraft, since that's the predominant manufacturer of aircraft currently in SwissAir's fleet. There may be subtleties from version to version.

Short answer: a blue light typically indicates a passenger has pressed the "call attendant" button above their seat. It's used to provide a central indication that assistance is required, and may help localise the row the passenger is seated in by only illuminating a blue light in some parts of the aircraft in closest proximity to the relevant row.



Longer answer

The panel with coloured lights to which you refer is the Area Call Panel (ACP). These panels are situated at either end of the cabin, and possibly in the mid cabin or at other locations depending on aircraft type and age.

The ACP typically has multiple colour indications, such as pink, blue, amber and green. Their meaning is as follows:


Pink: communication from cockpit. Normally steady, flashing if an emergency call is taking place (initiated in either direction: cockpit to cabin or cabin to cockpit).
Blue, steady: passenger call using at-seat attendant button in the passenger service unit (PSU -- overhead panel above each seat row). This light may only appear on particular ACPs, which enables the calls to be segregated by cabin and further by seat location.
Amber: assistance request from the lavatory. Steady indicates a passenger in the lavatory has requested assistance from an attendant. Flashing indicates a smoke detector activation.
Green: attendant call, using the cabin interphone to call between interphone stations.


The indication panel is integrated with, and receives event indications from, the Cabin Intercommunication Data System (CIDS). The CIDS is a crucial network on the aircraft responsible for handling crew communications, managing some aspects of the flight attendant displays, operating the speaker system and giving the relevant indications. Calls requiring attention from a flight attendant typically trigger a notification on the attendant loudspeakers and illuminate the relevant light on the ACP (unless already illuminated) in order to draw attention.

Furthermore, for passenger calls, the relevant seat number light in the PSU is illuminated when the attendant button is operated. This allows the request to be localised to the relevant row and side of the aircraft. If an aircraft door is open, the row number flashes -- typically a door open event leads to the PSU row indications to be illuminated to enable embarking passengers to find their seats more easily.



References

Only a small amount of information is publicly available about the ACP, its role in the CIDS and the CIDS more generally. The information here is corroborated by two sources:


A Lufthansa training manual for the A319/320/321 aircraft, published online. See pages 166, et seq.
Jonathan Hurst's site, in particular the excerpts of A320 technical operations manuals, chapter 10 "Cabin".


Any further sources or references, especially for non-A319/A32x models, are gratefully received in the comments.