I have fitted a new light to the bathroom. When the power is switched on the lights come on, when switched off the circuit breaker cuts the power to all upstairsw lights. The wiring is the old style, and I have made sure that they are the right colours ie blue & black
You have made a wiring error - when the light is switched OFF the light comes on and when you put the switch to the ON position it connects live to neutral with no load between them - a short circuit - the fuse /breaker prevents the wires from melting and a fire breaking out and the house burning down - which is good.
This link below has a picture that may help you - if it's too much for you then call in a sparky...
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix lighting.