A family of Microsoft word processing software products for creating web, email, and print documents.
… All I want is to undo that accidental action and write in black.
You haven't supplied a version for your Microsoft Word but I'm guessing that you have inadvertently turned on Track Changes. You should be able to toggle the option on and off with Ctrl+Shift+E or look for the Track Changes commands within the Review tab's Tracking group (Word 2007 and 2010).
If this doesn't resolve you problem, post back with any further information you may have and more assistance may be available.