iOS 8.3 Ignoring Inline CSS
"Smart" formatting (for lack of a better phrase) is added after the page is rendered and is not affected by CSS. Short of disabling the functionality completely, as shown in the other answer, you cannot affect it.

The reason the color works is because the colored text is part of the page being rendered - you are declaring during rendering that it should be black. The underline is not added until after the page is rendered, so you cannot affect it.