I think that is the low temperature for the coolant. I don't know if the battery packs use the engine coolant or a separate cooling system on cold mornings? I know the battery packs are not as efficient and can't discharge full range when too cold. I usually see the blue light on for a few minutes first thing on cold morning before turning off.
Dad - 19 MDX Sport Hybrid Adv Fathom Blue Pearl w/ Graystone
Mom - 18 RLX Sport Hybrid Brilliant Red Metallic w/ Ebony
06 TSX w/ Navi (Totaled Oct/19), 08 RDX Tech (Daughter), 11 MDX Adv/Ent