Black fire and white fire of Torah

התורה ניתנה באש שחורה על גבי אש לבנה.

The Torah was given as black fire upon white fire. (Yerushalmi Shekalim 6:1)

The Sefas Emes draws on the Chazal that the Torah was written in black fire on a background of white fire — the black being the visible letters, the white the hidden radiance surrounding them.

פי' בהירות התורה הוא נסתר כמ"ש מה רב טובך אשר צפנת ומקודם צריכין להיות במדריגת השחרות אחר כך זוכין לאש לבנה שהיא המנוחה:

The meaning is that the brilliance of the Torah is concealed, as it is written, "How abundant is Your goodness that You have hidden away" (Tehillim 31:20) — and first one must be at the level of the blackness, and afterward one merits the white fire, which is the menuchah (rest).

The luminous inner essence of Torah, the "white fire," is hidden and stored away. A person must first labor through the "black fire" — the toil of the revealed letters and the struggle of avodah — and only then does he merit the white fire, the restful, radiant penimiyus that lies beyond effort.

Summary: The Torah's "black fire on white fire" expresses two levels: the visible black letters that demand toil, and the hidden white radiance that is its true brilliance, "stored away" as Hashem's abundant goodness. One must first pass through the laborious stage of "blackness" to merit the "white fire," the menuchah and inner light of Torah.