Are you saying that or is Tumblr?
The colon in a URL like http://htmlhelp.com
is kind of a delimiter after the protocol (http). Tumblr probably refers to the account name or a directory name. It can't contain colon and it can't contain slashes because they have a special meaning in a URL, they separate directories. Period should be OK, but nothing stops Tumblr from limiting what's allowed as they please.