To get a best possible result with pre-keyed elements you have to color correct and composite them the right way and use blend modes. If you don't know what a blend mode is, I'm not going to go into a long lesson about this, but you'll find them in software such as Adobe After Effects, Photoshop and in some editing programs, for example Adobe premiere.

Adobe After Effects
If you are using After Effects you should already be able to find them, but if not you'll find them next to the layers name (Source Name) called Mode. If it's not there, click the Toggle Switches / Modes at the bottom of the screen.

Adobe Premiere
If you are using Premiere you'll find them by clicking on the footage on the timeline - Effect Controls - Opacity - Blend Mode.

It's not possible to say which blend mode is the right one because it can vary. But in general Screen and Add are good for fire and flames. For blood effects one would often use Multiply, but all of this should be done by experimenting.