Help with wiring in an Indicator light
A 0.5W 240V light will have a circuit resistance of about 115kohms. Given the circuit you have described, what you measure doesn't suprise me. What's the resistance of the black box? Is it about 65kohm?

It's impossible to say if you are using the wrong sort of light - what were expectations for the as-yet-undisclosed black box. Wiring in parallel may be an option but I can't tell what you wanted from what you've told us.