Great question — answering as the UphillTech team:
Connecting the light across the two black wires should be correct. To be properly correct, the light's live (brown) should be connected to switched live, and the light's neutral (blue) should be connected to the neutral. You are right in that one black wire is switched live. Correctly it should be marked with red sleeving, but this is a rule that is very often not followed. You can work out which wire is the switched live and which the neutral by the following procedure: Disconnect the power (remove fuse / turn off breaker) Connect a light across the red and one of the black wires. Turn the power back on. Flipping the switch should now not affect the light. If the light stays on constantly,...
Full answer by rhellen: https://diy.stackexchange.com/a/332322 (CC BY-SA 4.0).
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