I'm with dadiOH and Ransley on this one. Doing a thorough sanding job
on the existing flooring should have brought it back to it's underlying
color. At that point, the old and new wood should match pretty close,
or they never will. The old "It will match after it ages" is almost
always the sign of a hack.

To do this right requires years of experience and skill, which is why
you want a specialist. He either has to get wood that is very close,
or if that's not possible, know how to stain something with a grain
that is close so that it will match.

If I were faced with this problem, I would insist that the contractor
take a piece of the old wood and come back and show me that piece
together with a new piece, both of which have been refinished. You
will of course have to pay extra for this, but at least you won't wind
up with a mess.