To understand these lines from Doctor Faustus by Christopher Marlowe, you first need to appreciate the circumstance in which he finds himself. He has sold his soul to the devil in order to obtain knowledge and power forbidden to humans. The lines you are trying to interpret have to do with Faustus attempting to save his soul from damnation. The lines preceding your quote are the following: “The devil will come, and Faustus must be damn'd. / O, I'll leap up to my God!—Who pulls me down?” (Marlowe). Faustus is attempting to obtain forgiveness for his crimes against God, but something is preventing him from doing so, thus the fact that he is pulled down while reaching for God.
Now that we have covered the situation, let’s move on. Christ’s blood is a symbol of forgiveness. In Catholicism (which Faustus obviously opposed throughout the work) Christ’s blood is taken along with his flesh for Holy Communion. So one drop of the blood of Christ would be enough to save his eternal soul from damnation, but it is unobtainable for him.
You may also be wondering exactly what the “firmament” is. It comes from the Bible, and it is a barrier which separates heaven from Earth. So basically, Christ’s blood is flowing above the firmament, but since the firmament is a barrier, the blood, and therefore forgiveness, is unobtainable for Faustus. Basically, the lines are saying that there is no way that Faustus can save his soul.
eNotes also has some wonderful information about this work.