When you turn the light "off" and the fuse trips - you are actually turning the switch to the ON position - the wiring fault you have made at the ceiling has live and neutral connected directly together when the switch is On - with no lamp in circuit the trip operates.
When the switch is OFF the wiring fault you have made at the ceiling has the light permanently on - until you operate the switch and trip the MCB.
The two red wires should connect together and to nothing else.
Of the two Black wires - one of them is the neutral return path and should connect to the blue wire of the lamp.
The other Black wire is Switched Live - to turn the light on and off and should connect to the Brown wire of the lamp.
Your challenge is finding out which "black" is which. Switched live often has a red or brown sleeve on it.
You will need 4 separate connections
Live - two reds - nothing else
Neutral - Black & Blue
Switched Live - Black with sleeve & Brown
Earths - all together
Take care...
Peccavi
March 2011