Hyoushi - 表紙

扉（とびら） http://www.japanlink.co.jp/dtpjiten/pdf/pdf11.pdf

I assume you are talking about a blank page in front of the table of contents, but not the cover. The cover is cover(表紙) which you may want to put a jacket on it. I suppose you call that blank page "blank page (ブランクページ)" or "blank page before the table of contents(目次の前のブランクページ)". That must work and would be well understood.