invisible backlinks

A friend of mine has a PR4 Blog on a .edu domain. (the blogs home page is PR6, and the university's homepage is PR9). So I called him up and told him a little bit about what I'm doing and asked him if he'd put a link on his next post for me. I told him it'd help me get indexed and rank better. I also told him that the link could be just a period or a comma. He said no problem. I was thinking though, the link could also be invisible to make it less likely that someone will click on it and see that he's putting links on his blogs. remember its an university blog, and he really likes it, so I dont want to risk it for him.

<a href="www.mydomain.com">Hi</a> = link with Hi as the anchor text
<a href="www.mydomain.com">.</a> = link with a period as the anchor text
<a href="www.mydomain.com"></a> = link with nothing as the anchor text????? thereofre an invisible link??

Would the above code be an invisible link? and is it ok to do that? I mean, he's got a pretty authoritative position, I feel like google wouldnt be mad because theres one invivsible link there.

Thanks for the replies