If you were to stain a large, flat piece of BB ply with a pigment stain, such as Minwax, you would very likely see variations in absorption across the piece, which is often referred to as "blotching". This is because Baltic birch is a fine grained wood with tight pores. The surface does not absorb the stain evenly, so some areas will be very dark and others will look like there is hardly any stain in it at all.
On smaller pieces or pieces of fretwork, where you don't have large, contiguous surface areas, the blotching may not be all that noticeable.
You can use various pre-stain conditioners that are available, to help minimize the problem, but be aware that they will prevent the stain from absorbing as deeply as it would naturally and this will affect the final color. You might also try a dye. Dyes can take a little practice to achieve the color you want, so I would recommend testing on some scrap BB ply first, if you go this route.
