If both parents have blue legs, all chicks should have dark legs (blue or green.)
At least 3/4 of chicks should have actual blue (not green.)
If you get any chicks with green legs, you will know that both parents carry a recessive gene for yellow skin.
The genes involved:
Light skin is dominant over dark skin, gene is sex-linked (located on the Z sex chromosome. Because dark skin is the recessive, breeding two dark-skinned chickens will give only dark-skinned offspring. "Dark skin" makes the shanks (lower legs) be either blue, or green, or black. Other genes determine which of those three it is. The soles of the feet will often be white (with blue shanks) or yellow (with green shanks).
White skin is dominant over yellow skin. This gene is not sex-linked. For light-skinned chickens, the shanks will actually be white or yellow. For dark-skinned chickens, the shank color changes but the soles of the feet will often show white or yellow anyway: blue with white soles or green with yellow soles.
There are other genes that can make the skin actually black (like in Silkies and Ayam Cemani), and those can affect the soles of the feet and many other parts of the body as well. But they are not relevant to your situation.
If each of your chickens has blue legs, that means they each have dark skin (recessive, should breed true when paired with other dark-skinned chickens). They each have "white" skin (not yellow), but because white is the dominant trait you cannot tell whether either of them is carrying the recessive gene for yellow skin. If they are each carrying the gene for yellow skin, then about 1/4 of their chicks will have yellow skin (green shanks, because they will also have dark skin.)
Alternate words for some shank colors:
slate = blue
willow = green