How can I delete an email attachment that is open in mail on my iPhone and blocking my ability to see other emails.

Have you tried tapping on the screen to get the Done button in the upper left corner? When you tap on Done, it closes the attachment. If that doesn't work, force mail to close.

Press the Home button two times quickly. You'll see small previews of your recently used apps. Swipe left to find the app you want to close. Swipe up on the app's preview to close it.

Then launch Mail again and see if it is gone.