If you want to say ‘A blot on the landscape’ in French, you should say: ‘ Tomber comme un cheveu sur la soupe’. Literally this translates as: ‘To fall like a hair in the soup’.
And for ‘To carry coals to Newcastle’, say: ‘Porter l’eau à la rivière’. This translates literally as: ‘To carry water to the river’.
‘Être à cheval sur les règles’ literally translates as ‘To be riding on top of the rules’. We would say: ‘To be a stickler for the rules’.
Taken from: ‘Don’t toss Granny in the Begonias: Faut pas Pousser Mémé dans les Bégonias’ by Marie-Hélène Claudel-Gilly and Primrose Arnander.