The level size felt good.
The blood levels felt fine but I had a difficult time noticing the bar changing while I was running through the levels. It may help to have a different color (maybe just a darker gray) underneath the red bar to make it easier to see how much is missing.
I'm still not sure what the purple vials do. Even just adding a little description for everything on the pause menu would be great. As for powerups, it would be cool to have some kind of timed spectral power that allows you to walk through enemies for a short period.
The horde was great. It's visually clear so it's easy to spot as soon as it creeps up on screen.
The score definitely helps with replayablity, especially with people sharing their highest level (16 for me).
I think a max blood power up would be good but there also needs to be more ways of getting blood. By the double digit levels, I don't think I ever had a full blood level.
Additionally, I think the obstacles and hit boxes are a little too unclear. Either changing the color of obstacles to something more distinguishable or adding outlines to obstacle sprites would help with that!
Great game! I love the art and mechanics, it all fits the tone so well.