On this item I had entered a snipe well in excess of the final winning bid, but it was apparently not applied. The item shows up in my BidSlammer history in blue with a sad face. The dashboard keys blue as “set,” but I can’t find an explanation of what that means or why my snipe failed.

Hello Peter,
Thank you for your question about this item. We investigated to see what we encountered when we placed the snipe for you.
On the free plan, the bid is submitted only one time from one location, so we don't have multiple bids to compare results. For premium snipes, we know if it is eBay or account-related. we know it is eBay- or account-related.You are on the free plan.
We have maintained a list of possible reasons over the years. Please check this list to see if any of the circumstances apply:
https://bidslammer.com/help/bulletproof
Hopefully this helps. Please reply if you have other questions.