I am a memory made of light, captured once, then held tight. What am I?

Before theory, I might explain everything. Before along, I might mislead you. Before instrument, I might be tucked under a chin. Alone, I’m useful or annoying.

I make blending ingredients easier, and sometimes even hands free. It's also essentially what a dj is.

I go in dry and come out wet. The longer I'm in, the stronger I get. What am I?

A school’s paper companion, A scholar’s delight, An artist’s tool, And a banker’s nightmare.