In cases where a number appears in an element whose rendition is “slant(italic)”, the WWP will not indicate any special rendition for the number, even if it appears to be in a roman typeface. This is because in the WWP’s texts, there do not appear to be any instances of italicized numbers, so that the use of the roman typeface for numbers does not indicate any choice on the part of the printer, but only the absence of any other option. For this reason it seems unnecessary to highlight what is simply the only possible rendition.
If any case does arise where a number is italicized to differentiate it from the surrounding text (to give emphasis, for instance), the WWP will treat it just as we would any other italicized element:
I could not believe my ears: <emph rend="slant(italic)">15</emph> eggs in one day from a single chicken.
Numbers in blackletter should be treated like any other text in blackletter.