I will say it again;

The wiki has the dec code of : 4 for lapis dye

If you put that code on the sign, it is for cobblestone.

The page I provided, gives a correct list of the entire code for specific items.

Please read my original post if you wanna compare the code '4' to what lapis actually is. The signs recognize it only as 'blue ink sac' and not 'Lapis Lapizuli" which is it's item name.