I think your hen who lays blue eggs, with a Welsummer rooster, should give at least some daughters that lay nice olive eggs, probably freckled.
That assumes the Welsummer rooster has the right genes for dark brown eggs with darker speckles on them (probably true, but since he doesn't lay eggs, there is no way to be sure until you see the eggs his daughters lay.)
A different breed of rooster will not help. A different individual Welsummer rooster might be better or worse, but you won't know without raising daughters from each one and comparing.
From this hen, if you skip the DNA test, you will not know whether to expect olive eggs from half of her daughters or from all of her daughters. But since you know that she does lay blue eggs, that means she has at least one blue egg gene, so at least half of her daughters should inherit that. If she really has two blue egg genes, then all of her daughters will inherit a blue egg gene from her, and they will all lay blue/green/olive eggs (olive in this case, because of the rooster being a Welsummer.)
If you want to sell chicks at hatch, and tell the buyers "all chicks will lay olive eggs" or "half of chicks will lay olive eggs and the other half will lay brown," then you might want to get the DNA test on the hen so you know which statement is correct. If you want to hatch chicks for your own flock and you are happy to get some olive eggs, with a chance of some laying brown, then I would just use this hen and probably not bother with the DNA test (unless you are curious and impatient, in which case you might be willing to pay for the test just to get your answer sooner. I've done that a few times myself.)
