Hello,
If you complete the main quest before you complete the Vampire Cure quest, you may be unable to give the bloodgrass to Melisande. To workaround this, reload a save from before you completed the main quest, then finish the Vampire Cure quest first. If the problem still occurs, start a new game.
We are aware of this issue, and it has been entered into our database. However, at this time, there are no planned updates at this stage for any version of Oblivion. Please check the website for any announcements if/when that changes. The game can be successfully completed without finishing this particular quest.