Both of those apps will NOT check for existing duplicates without editing the records, so they won't do the job.
If something as simple as email address is the thing that identifies your duplicates you can use this excel trick. http://crmcheatsheet.com/how-to-de-duplicate-data-for-import-to-salesforce/
An app that is really good for checking existing duplicates is Demand Tools (by the same company that make dupe blocker). It is a windows app where the data is download, scanned for duplicates, you check through the results and tell the app which records to merge. It is pretty difficult to get your head around using it, and still tedious to dedupe, but definitely worth the money (and it's free for NFPs).
Deduping is hard. It's meant to be. It's meant to shock you into the reality to never let it get that bad again, and you can use the tools mentioned above to manage them ongoing or something like Ring Lead to stop them getting in there in the first place, or wait for the native dedupe feature to be released in an upcoming release soon.