It's not really clear.
I don't think they are illegal because the law doesn't say they are, but cops do think that double-edged knives are illegal. It seems that they use the term "dagger" to mean any two-edged knife...which is a perversion of the definition at best.
A knife with a gut hook is a two-edged knife, but no reasonable person would think that meant it was a dagger. I don't think a clip blade with a sharpened swedge is, either, but I don't want to be the test case just to prove it.
So, I'd say you're better off to get one with an unsharpened clip swedge.