Marcros is right, steamed ABW is normally sold on a "sap no fault" basis. Having said that the timber yard has dumped on you the stuff rejected by all the buyers who selected in person, and the stuff the yard daren't send out to their bigger customers. It's not technically out of spec for ABW, it's just the bottom of the barrel.
There are a very small number of timber yards in the UK that sometimes have waney edged, unsteamed ABW. And if you're lucky enough to be there in person when a fresh delivery arrives you can pick through wide boards and find gems like this,
Unfortunately sap will always be an issue with the majority of ABW, hence steaming which minimises the colour difference, although at the cost of dulling down the grain and leaving it a bit flat and lifeless. If the colour difference is still too much then the best remedy is generally a coat of dilute stain across the entire piece followed by additional dilute coats just on the sap sections, feathering these into the heartwood. Use a water based, dark Walnut aniline dye. It's not perfect, because aniline dye will fade in sunlight, but don't be tempted to use spirit based stain because of the "lightfast" claim, it's a tiny, tiny bit more lightfast than water soluble but far, far more complex to use because it dries so fast that you can't retain the wet edge for feathering. Unless you've got plenty of practical finishing experience under your belt stick to water based. You really don't want to mess around with
pigmented stains, the grain on unfilled Walnut is just too coarse so you'll get lots of dark flecks which looks like complete carp.
There was another thread recently about Walnut which you might find useful,
british-walnut-prices-t106138.html
Good luck!