In order to full grasp the reference, the entire quote has to be analyzed. Consider Eliezer's words very carefully:
The night had passed completely. The morning star shone in the sky. I too had become a different person. The student of the Talmud, the child I was, had been consumed by the flames. All that was left was a shape the resembled me. My soul had been invaded- and devoured- by a black flame.
At this point in his experiences, Eliezer's statement reflects one of the first moments where his change becomes profound.
I think that image of fire is not a misleading one. From the fire of the candle that accompanied Eliezer's studies of the Kaballah and his searching for a guru or spiritual master, to the flames that Madame Scachter had been screaming about on the train, to the experiences of seeing the chimneys upon his arrival into Birkenau, the presence of fire has accompanied Eliezer through his narrative. In the idea of being "devoured," Eliezer begins to indicate that his faith in God is disappearing. While he is nowhere near the bottom of human cruelty and as things will progressively degenerate even more, Eliezer's reference to the "black flame" also indicates the dehumanization of the Holocaust. When fire has torn through an object, all that is left is the carbonized and blackened remain. Eliezer is suggesting that he has become this remain in that his humanity has been extinguished. In some respects, the threat of the crematorium is one made in vain, as Wisel seems to be suggesting that the Holocaust's true horror is that it killed people without actually killing them. This emotional carbonization is what indicates the change within Eliezer, and helps to bring out the idea of a "death" before actual "death." It is from this point where Eliezer's relationship with God, or the lack of it, is going to be critical in the formation of his identity.