Blue joke about rubbish church (7)

I believe the answer is:

obscene

I'm a little stuck... Click here to teach me more about this clue!

'blue' is the definition.
(blue can mean pornographic)

I don't understand the rest of the clue.

(Other definitions for obscene that I've seen before include "Rude" , "Improper" , "Shameless" , "off colour?" , "Bawdy" .)

I've seen this clue in The Independent.

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