In Pri Etz Hayim, Shaar Hatzitzit, Ch. 4 (pp. 58-59). He explains that in the time of the Mikdash they had 7 white and 1 tekhelet on each corner. He says that from a kabbalistic perspective tekhelet represents malchut and thus we would really only need one string on one corner – i.e., 31 strings of white and just one tekhelet, but he says this is not the way to do it, rather we put 1 blue and 7 white on each corner.
– Mois Navon.