If you seed or turf then the borders are going look very good and will stand out against the rest of the lawn, even more so if your lawn is of poor quality, I have always found that given time it gradually blends in.

You'll need to get the levels right too. Remember to firm the new area - you can use a board or something you can walk over to get a firmish surface. Then turn the board round so half of it is over the border and half over the lawn and you'll be able to see where you need to add or remove any soil. Check the quality of the lawn grass - whether the blades are fine or broader. Broader ones will mean you want a seed mix containing rye grass which is hard wearing. fine ones make a beautiful lawn but less touch for playing on. They will be slightly different shades of green so if you match the seed it will blend better. Most houses will have the rye grass mix.

Agree with both answers - can take a couple of years for the new grass to start looking like the pre existing grass. The most difficult part is getting the levels to match up - what looks like a matching level initially often drops after a while.