I am not certain about suretrader but in TOS there are green, red and grey. I am thinking the blue on your chart is the same as grey on mine. If so I was told that this is b/c they buy and sell where both the same. Ex. you buy 500 shares and I sell 500 shares at the same time. Google volume and what is it actually telling me. Thats my guess.

Same on DAS trader, blue is equilibrium (buy and sell same, 0 move) the handshake between buyers and sellers and a good sign of trend change or a place for future res/sup level

Thank you both for your input!