Need cake icing help, complete novice alert!

[MinkyBorage]

I'm making a spooky castle for dds halloween birthday, with black sugarpaste. This is probably the first of many stupid questions, but if I dust the paste with icing sugar whilst rolling out etc, won't it spoil the blaco colour?
Also, I'm going to try to buy cake smoothers tomorrow, but if I can't get any, is there anything else I could use?
I'm making a chocolate fudgecake with buttercream filling. How long is it likely to last. I was thinking of making the cake tomorrow and covering it in icing asap, the party is on Saturday. Will this be OK?
Any other tips or advice would be fabulous.

*black

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I make this chocolate cake a lot and it lasts ages. At least a week. Can't help with the icing.

When I've used black sugarpaste before, I've gone over it lightly with a wet pastry brush to get rid of white marks. Hope this helps!

Black sugarpaste does get a little discoloured from icing sugar but as I've never found an alternative it's just something we have to live with Although black sugarpaste always looks more gray when rolled out anyway.

Palm of your hand is an ok alternative to a smoother if you aren't used to using one but nothing beats the real thing.

I'd make the cake (yummy!!) tomorrow, fill it, and leave it in the fridge over night. Ice it on Friday. Once it's covered in sugarpaste don't put it back in the fridge.