Thanks everyone for your comments. I have tried to wash the blocks, and could not get any of the stains out. Also I can't match the colour with new for the blocks that aren't done yet, so will replace it all. I will post a picture when done, but it could be a bit yet.

Depend on if you like that look and like to do hand work.

My mom had a number of embroidered pieces from the 30's and 40's that had the black outline stitching.

My MIL made lots of quilts and I think she did either white or matching stitching on her SBS.

Many years ago my mom made us a 25th anniversry Sunbonnet Sue quilot and she used colors that matched the dresses and shoes. It is delightful.

You might tea-dye the rest of the muslin if the stain is just from age.