I use blog comments for diversification purposes and even nofollow ones work very well in that purpose. I basically search for related blogs manually, read their post, and then leave a comment that actually makes sense with my name (persona for the site I'm targeting) as the anchor. I've been looking for such services as it's time consuming for me to scale it for 5-6 sites together, but in vain. The majority of services here offer the spammiest of blog comments on pages with hundreds of OBLs and contextual anchor rich links that are enough to invite penalties on their own. If you know of any service that's likely to fulfill my needs, please let me know on this thread.