You should iterate over the buffer and collect your text (untested):
(defun get-paragraphs ()
(save-excursion
(goto-char (point-min))
(let ((ret '()))
(while (search-forward-regexp "^: " nil t)
(let ((header (buffer-substring-no-properties (point) (line-end-position)))
(body '()))
(forward-line)
(while (not (looking-at "^$"))
(push (buffer-substring-no-properties (point) (line-end-position)) body)
(forward-line))
(push (cons header (list (reverse body))) ret)))
(nreverse ret))))