A bit late to the party but the spambot above me threw this one in to the light again.
National union of journalists have a guide to free lancing but obviously the prices aren’t realistic in this scenario. http://www.londonfreelance.org/feesguide/index.php?§ion=Photography&subsect=Online+use+of+photos
In a case like this, might it not be better to suggest you do a swap for time? It may well be that both of you come out better off having bartered services with each other instead of money changing hands. It depends on the blog a bit though. If it is one with 100k clicks a day I suggest you charge her according to the NUJ rates but otherwise you might find it beneficial not to. Could you business benefit from having a MUA and thus bring in more money that way?