You could use a bit of wide eyelet lace - it wouldn't take much. If you can't find lace wide enough you could sew 2 pieces together along the long edge and then attach it to your blouse.

Donna

Remember dickies??? You can make your own using an old shirt/blouse.Just cut off the sleeves and the bottom,both in the front and back.I make the back shorter than the front.
Kathi

I have used lace doily inserts to make a plain v neck into something dressy. Joanns always carried them but don't do if they still do.