One thread of blue on a corner. Read plainly, that is what the verse asks for, and that is what this is. It is not rabbinic tzitzit and does not replace it: this is our own reading of the words, not a ruling, and anyone obligated in tzitzit should keep wearing them.
Colour note: the blue used across this site is a placeholder. It is pending a physical dye-match test against real techelet and is not the final colour.