Careful with the hint translations, when you see (I) in front of a word, it indicates a verb. In this case, (I) leaf (through a book). That's just a suggestion for the future... but we will fix this as soon as possible, as the verb to leaf is more commonly translated to sfogliare not fogliare :) Thanks for pointing it out!
According to the big Hoepli dictionary, "a blank page" is una pagina bianca, and both the Anglo-French idiom "carte blanche" and the idiomatic English "blank cheque" are carta bianca. However, most other uses of "blank", including sheet, form and a real paper cheque, use ... in bianco.
I wonder why; what's the rule? Maybe something intended to be filled is in bianco?
No; without attributes it only means a sheet of paper; it can also refer to a thin layer of metal, plastic or wood in the appropriate contexts, but that meaning is less broad than the English sheet, and in many cases where you'd use that Italian uses the more literal "strato" (layer). Bed sheets / linen are "lenzuola".
No. There are a few problems with this.
"It's a clean sheet" could refer to any kind of sheet - bed sheet, plastic sheet, etc
While from what you're meaning to say, they do mean the same thing, we are supposed to translate it as literally as possible while still making grammatical sense. So "It's a clean sheet" or "It's a blank page" is counted as incorrect, because the closest literal translation is "It is a white sheet of paper."
Or "it's a sheet of white paper", because the sheet or the paper can be colored... "è un foglio colorato" (it's a sheet of colored paper or it's a colored sheet of paper). Anyway the DL sentence as usual doesn't explain anything ("foglio" can be made of different materials), so we have translate as we think it might be better, hoping that DL accepts the translation.