Why can Alpine glacier ice look blue in a cleft but white when broken?

Across an Alpine glacier, the ice could appear blue, from a faint tinge to a deep hue; the blue was described as especially vivid in clefts where the ice was thawing^[1][2]. Yet a crystal or small piece broken from the mass could look simply white and transparent, or frothy and semi-opaque^[3]. The account introduces these colours as something travellers to Alpine glaciers spoke about broadly; it does not assign this particular description to a named observer or a mountain pass^[4].