Letting blood sit inside your syringe will cause the blood to coagulate/clot and will render your syringe and solution un-injectable. If you get blood in the syringe, you can squirt it back out into a clean spoon and then refilter it back into a fresh syringe if you plan on using it immediately. Otherwise you could drink/squirt it into your nose if you are desperate. Injecting clotted blood is extremely dangerous, having a blood clot flowing thru your circulatory system can cause extreme complications.
Can you not hit a vein because you are new to IVing? If that is the case let us know so we can provide you with some links to threads we have on here regarding proper IV techniques and harm reduction material in order to minimize any potential harm you are doing.