When you speak to several NPC's at varying times they will give you a vial of their blood. They seem to just be additional healing items but they also appear to be too special for that to be there only purpose. So is there any point in the game where they are traded for something or affect gameplay in some way?
Primarily you're right in that they're meant to just be additional healing items, generally with special qualities compared to your regular blood vials (heal-over-time, increased healing amount, temporary buff to stamina recovery). There's an interaction between a couple of NPCs depending on which special blood vials you decide to take:
If you've rescued both Adella and Arianna (via Oedon Chapel, not Iosefka's Clinic), you can get special blood vials from one or both of them. Receiving blood from Arianna three or more times causes Adella to become jealous, and she'll kill Arianna the next time you load into the Cathedral Ward lamp area. This has later implications for the storyline, with the general consensus being to try and keep Arianna alive.