Simple filter to unhardwrap multiple paragraphs

I have many markdown files which are hardwrapped to 80 characters. I would like to copy these to paste into textareas (xclip -sel clipboard < file), but want them to wordwrap normally. This is simple w/o paragraphs - tr "\n" " " < file | xclip -sel clipboard, but turns all text into a run-on paragraph which still requires manual copy-editing. Is there a short and simple filter using standard *NIX tools (GNU versions are fine) to delete all "\n" _except_ when there are two next to each other? Ideally something I can remember and retype - less than 30 characters with no files. It feels like something sed should be able to do.