Are there any wires hidden up inside the ceiling? With only one red, two blacks and one earth I can‘t see how the bathroom light would be affected.
The most logical explanation (but by no means the only one) would be that red is the switched live, the blacks are neutrals and two permanent lives are linked out of side.
You need some kind of measuring device (multimeter or two-pole voltage tester) so you can measure which is which.
Measure between red and earth with the switch in both positions. If you only get 230 V (or thereabouts) with the switch in one position, red is the switched live. Then measure from red to both blacks, one after another, while the switch is one. The black that gives you 230 is the incoming neutral. If that‘s what you get, connect both blacks to the blue on the light and red to brown. Earth to earth.
If not, you might need to get an electrician involved.
The most logical explanation (but by no means the only one) would be that red is the switched live, the blacks are neutrals and two permanent lives are linked out of side.
You need some kind of measuring device (multimeter or two-pole voltage tester) so you can measure which is which.
Measure between red and earth with the switch in both positions. If you only get 230 V (or thereabouts) with the switch in one position, red is the switched live. Then measure from red to both blacks, one after another, while the switch is one. The black that gives you 230 is the incoming neutral. If that‘s what you get, connect both blacks to the blue on the light and red to brown. Earth to earth.
If not, you might need to get an electrician involved.