According to ghacks, Mozilla wanted the feature removed from about:config and only re-settable via extension:
The
browser.newtab.urlpreference has no exposed UI, is not really supported, and is abused by search hijackers. We should remove it and encourage people using a non-default new tab page to install an add-on instead.
So, unless policy reverses itself (unlikely, as this is not the first time Mozilla has removed about:config features and insisted they be replaced by extensions), using an extension like New Tab Override is the only option.
UPDATE: If you install New Tab Override, you must configure it first, and manually add about:blank to the extension settings.