The top plate should have grain as straight as possible, since to top is the acoustically most important side of the instrument (it is some sort of membrane, moving the air). The back does matter much less. This is the reason why the top is made of a very straight growing, light and soft wood (spruce), while the back is made from a dense, hard and robust material (sycamore (or outside of europe often maple)).
So probably this will be fine acoustically.
The heavy accent on the grain (and also the coloring of the flame) is caused by directly varnishing the wood with a dark varnish or a varnish with pigments. You need to keep in mind that the grain will lead to parts that will absorb liquids differently well. If you then use any form of stain the parts that absorb the varnish better will accumulate lots of color, leading to darker spots. The dark line is merely a part of the grain that drank a lot of varnish.
This is usually considered unwanted, especially as it ruins the amazing shine you get with figured wood (which is caused be the rippling of the grain reflecting lights at different angles). Of course on the other hand you might be able to get an amazing accent of the grain this way, but usually this is not what we want to go for.
So what is done instead is that the instrument is first coated with either very light varnish or hide glue, sealing the wood. This will then mean that further coats of colored varnish will not penetrate the wood and will form a layer of even color. Also due to the wood not actually being stained the wood remains reflective, giving you the typical shine.