Q. I washed some off-white 90% cotton blend towels by themselves with bleach and Trader Joe's Detergent. They came out with a blue cast to the base, and are almost black below the loops. Why did this happen, and is there anything I can do to correct it?

A. This is somewhat puzzling. I suspect that the towel is probably made of cotton loops and a synthetic base so that the absorbent cotton can do its job.

Curious if you had ever bleached the towels before without this problem occurring? If so, what was different this time (the products, the temperature, etc.)?

What may have happened is the different fibers may have different brighteners applied during their production. Brighteners are used to enhance the white appearance of an item. The synthetic brighteners may have reacted with the bleach and changed to a blue/purple color. I'm not sure if this is reversible or not. You might try a quick wash in diluted vinegar to see if the pH change will switch the color back. If not, then I'm afraid it's a done deal.