Most online tzitzit vendors that sell blue threads, either Radzyn or Ptil Tekhelet (Murex trunculus) sell only all blue strings. But we also offer them with half blue/half white strings. This option tends to confuse our customers a bit. Here's a recent inquiry on the topic:
I would like to order tzitzit strings for my son's bar mitzvah. He
wants 1 blue thread. Do I order the Radzin Techelet Tzitzit Strings, 4
blue and 12 white, thin strings, all blue strings? Thank you! - Celia
tying tzitzit, you always start with four strings on each corner. After
you insert them, you even up the sides, tie a double knot and then get
started tying the tzitzit according to whichever custom you follow.
So a lot of people get the idea there are eight strings on each corner. You see eight dangling down, but they are really four.
question is does he want one whole blue string or one half blue string
on each corner. If you use a whole blue string, in the end you see six
white and two blue. If you use a half-blue, half-white, then in the end
you see seven white and one blue.
white and two blue is more common. Many Sephardim and Chassidim who
follow the Arizal and/or Rambam may go with seven white/one blue