This can be VERY dangerous and is not recommended. But here goes anyway.
One most important turn off the power at the consumer unit use main big switch I know there is a small one for lights alone but it does not switch the neutral and depending on earthing type neutral can be live so being super safe. Two turn mutimeter to VAC if a range then above 250 volts and put one lead on earth and check all reds then one lead on red and check all blacks. OK now we are reasonable sure there is no power as any power will likely damage the meter. Just in case you have switched off wrong main switch or there are two.
Now turn meter to ohms range Ω and if it has buzzer all the better. There is no real need to zero if using an analogue meter. Put meter across each pair of wires and then switch the light switch. When you find correct set you will get two different readings with switch off and switch on. Then mark the Black/blue wire with insulation tape or sleeving.
Yes you could test live, but both feed to next lamp and switch wire will show dead so far easier to use ohm range.
Very VERY carefully!
A multimeter must not be used on live circuits.
to identify switch line with circuit dead, connect a length of wire from the switch black long enough to reach the centre light.
connect one wire from the m-meter to it, use the other meter lead to touch each blk in turn with meter selected to ohms/continuity range.
one blk will give full scale deflection of needle if using a 'proper' moving coil meter,
or if using a digital type, it will read a high number untill touching the correct wire when it should go to (very nearly) zero.
once sw'line found mark it with sleeving/tape to show that it is really a 'live' conductor, also at switch end.
disconnect all test wiring as soon as possible, it's easy to forget,
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