Unless you use black flour it won't turn black.  It will be darker in color than the flour you use, though.  I often mix bleached cooking flour in with mine if I want the colo

Black lines are typically indicative of water getting to the wood rather than the epoxy darkening, at least in my experience.

r to be lighter. 

FrankP

Stupid editing window broke my sentence. 

You could make some wood flour out of strips of the same color. Or if the gaps are wide enough, shave some thin slivers of wood from a similar strip, and wedge them in where necessary.

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Ogata

You could make some wood flour out of strips of the same color. Or if the gaps are wide enough, shave some thin slivers of wood from a similar strip, and wedge them in where necessary.

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Ogata

In my experience, wood flour of the same species as the surrounding wood will cure somewhat darker when mixed with epoxy. If that is objectionable and you want a match, try using some sanding dust from a lighter-colored species of wood experimentally before you commit to your boat.

 Jim

I've covered off-color seams using model builders paint applied with a fine pointed brush with surprisingly good results. -Wes

My experience is similar to Jim's...when I made wood flour from the same type of wood I was trying to match, it came out way too dark. Next time I'll try something else.

Mark

I did a little of both. Some places I needed to wedge in some pieces of cedar and other places it just needed filler. I usually mix (50-50) sawdust from the cedar with silica to make a lighter colored mix since it does darken. It comes out better but I'm not very scientific with it.