sorry for telling just i did with my own site for checking that negative seo is working or not
I created alot of xrumer forum profiles with main keyword,adult keywords and some keywords like cheap viagra. I Created more than 200,000 Verified backlinks also for indexing
Then tier2 posting with same content. Next step was to index again all backlinks with paid indexing services like indexification,incredible indexer.
All keywords rank up for one week all on top and after two weeks all keywords kickout from google So my opinion is negative seo is working.
Don't blame me this is just You can say I Just want to be make sure that negative seo is working or not.