husband would be worth asking about this - he knows a fair bit about optical brighteners, etc. Since they began adding those to laundry detergent, "blue bag" went out of fashion.
You may find, Fran, it mightn't make the difference, especially if the laundry detergent you've been using has optical brighteners in it.
If you find the blue doesn't make enough difference there are still other things you can try. Drying it in the sun, especially after a rinse which includes some form of dilute acetic acid (watered down white vinegar or lemon juice) can help.
Depending on what the yellowing is from, here's a trick you can also try - splash the yellow areas with neat white vinegar. Then soak the garment in a laundry soaker (a lot of detergents can double as pre-soak) using water as warm as the garment can stand. If it's pure wool, cold water will do, just you might need to repeat this. Then wash as normal, hang it in the sun to dry (unless, of course, it's pure wool - we find even sunlight shrinks wool, especially in summer - and your summers sound a lot like ours).
This works especially if there is any protein component to the stain - perspiration, for example, can seem to have been washed out completely, but it leads to a garment yellowing with age, in the patches where the perspiration once was.
Good luck with it - and if husband has any more expertise to add, let's hope he has time in his schedule.