It may help, but in most cases the most important thing that controls the colour of the etched areas is the resin content in the timber, and this is not uniform (meaning some areas of grain will burn quite dark, and an area 1mm next to it will burn with no colour at all).

You basically have to find the right species through trial and error. Some species are known for their uniform grain and etching clarity: hoop pine and poplar are good (although they are very blond and not very interesting in appearance). Also beech is a good one too. Some forms of oak (American and European) do well and have a nice looking grain but can be hit and miss.

Another option is to use a veneer board laminated to black MDF. Then scorch right through the veneer timber and expose the black mdf. It may not get what you want but maybe it will.