I usually do algebra tiles as a tactile graphic. When they are coloured, I use a key to describe the colours and textures. These are usually differentiated in print as positive and negative tiles by their colour. The students get used to the format and understand the meaning. When necessary, I may insert a key as a reminder if there haven’t been any tiles used for several chapters. I would not use a key every time they occur. If there is only 1 colour in a set of examples, that could be explained in a TN rather than a key.
Since your worksheet doesn't seem to explain the significance of the black & white tiles, I think I would use just an outline for the "white" tiles, and use a textured fill for the black tiles. Make them proportionate in size as they are in print.
I will also attach a document that we have used to explain to transcriber's how to show algebra tiles. Since they should be shown as a tactile graphic, you could also ask for assistance under that area of the forum.
Hope this helps!