I found a solution. Blynk has an alternative format of ?pin=longitude,latitude.
So a full URL for sending updating a location datastream ‘V7’ would look like this:
The HTTP request sent to the webhook URL still looks garbled, but it is accepted by Blynk:
GET /external/api/batch/update?token=AbcdefgHIJklmnopqrstuvWxyZabcdeF&V7=-73.987991%2C40.725418 HTTP/1.1
User-Agent: ParticleBot/1.1 (https://docs.particle.io/webhooks)
host: ny3.blynk.cloud
Connection: keep-alive
Thank you for your help and patience.
If anyone else needs to do this, I am posting all of the details to my website:
https://savvymicrocontrollersolutions.info/particle.php?topic=particle-blynk