As an offering to Diana, a good start could be moonwater.
http://www.witchipedia.com/def:moon-water
If you insist on using blood as an offering, I would suggest the blood of life(menstrual blood) as opposed to blood that is shed in a sacrificial manner, simply because it is more in relation to what Diana herself operates in.
As you become closer to the goddess, and actually talk to her, and more importantly, listen to her, intuition and her own words will guide you to a proper offering.
A glass of water, or a white candle is generally accepted as a universal offering that any spirit will accept, and is good for when first begin to work with the entity, before learning what their preferences are.
Despite this original post being quite old the recent comments on it are still relevant to individuals seeking out ideas today. I personally don't believe using blood in a spell makes it more powerful or better, it all comes down to your connection with what it is you are looking to accomplish.
I like to use a drop of blood in some spells, only for that personal life-force feeling. Blood is part of you and your desires and will compose of your connection to life. Adding your own blood would only aid you in what it is you are looking to achieve, however if you want to just use a substitute it won't make the spell any less effective as long as your will and mind-set are focused.
Just use caution when using your own blood. I happen to have a handy-dandy blood glucose meter with disposable sterile needle points. This makes it very easy for me to prick my finger but for someone that does not have this tool make sure you are using something sterile and always wash your hands before if looking to prick a finger. Then clean or dispose of the needle properly, as always common sense when dealing with things like this.
Again blood is a choice and really personal feelings. You can use dragons blood and often I use this in place of real blood when writing spells out. There are several ways to create a usable liquid-ink using dragons blood to get that same effect.
