There is scope for confusion here. I would make sure the price has been clarified, per blanket, to the buyer before the blankets are handed over. Also keep your receipts for the wool in case of any query or raised eyebrow, even though the fixed price was calculated with direct reference to the price of the wool. Be ready to slip this in discreetly with the blankets, if you think your colleague may have been misled by previous conversations. It is surprising how little people who don't knit know about the price of wool, and what others might think is obvious. If you haven't got receipts Google the current price and number of balls needed to double check, as a miffed buyer might do the same.. Your time was given freely, and I can not see that there are any grounds for your colleague to take umbrage even if they did think you meant £20 for both, as I doubt they would expect you to subsidize the blankets out of your own pocket.
I had something happen to me,
years ago but with the boot on the other foot, when I got married. My MiL to be offered to do the flowers for my wedding bouquet as she was an ardent flower arranger. I thought it would be rude to say no, so I cheerfully accepted. I was a bit taken aback when she expected me to pay for the flowers and accessories, as I genuinely thought she was offering to do them for free. She didn't have a lot of money so I accepted it was my mistake, but how I'd wished that I been able to choose my own flowers from a florist, and had exactly what I had in mind, rather than her interpretation. Lesson learned.