They might have messed up there I guess. But where Again are we told that Blue is not able to tell lies? and by whom? I dont remember that.
Sherazade, in S1x19 The Return, after Bae disappears through the vortex and Rumple is left alone and he calls on Blue, they can’t into an argument. Rumple keeps asking her if there’s a way and she says no (because she’s used the last magical bean). He calls her a liar and she says she’s not lying because Fairies can’t lie.