It is common for kilns to steam walnut. Steaming helps the color of the darker heartwood color bleed into the lighter colored sapwood making the walnut a more uniform color.
When buying steamed walnut, most companies note on their PO's “well steamed”, But this is misleading. The color is affected by the region where the walnut is grown and how long it is steamed. To prevent misunderstandings and possibly losing a good supplier, be clear about your specific needs and what you expect when you say “well steamed”. To assure that the walnut is in the color range you require, ask for pictures of the load prior to shipping.