I just ran into the same exact issue, and found the answer! If you are building the Cordova/PhoneGap project locally on your machine and are NOT using PhoneGapBuild, the fix is really simple.
In Xcode, open your project, expand the yellow Classes folder and open MainViewController.m file.
On line 107 of MainViewController.m you see the line below :
theWebView.backgroundColor = [UIColor blackColor];
Change it to :
theWebView.backgroundColor = [UIColor whiteColor];
Alternatively, if you are using PhoneGapBuild and don't have access to modify the iOS project files, someone developed a cordova/phonegap plugin to fix this issue. It can be found below.
https://github.com/EddyVerbruggen/iOSWebViewColor-PhoneGap-Plugin