How can I use these fan blades?

Guess you could see if the manufacturer of your fan makes other blade choices for the fan model that you own. There are also fan blade covers now available. I think a different paint would lighten up that brown color. I’m with you on modernizing those blades!

Not sure if you are upcycling or just want to change the look.

If it's changing the look, then paint each one a different colour, to complement your colour scheme.

If upcycling then turn into garden art and turn them into wing for dragon flies or butterflies.

https://www.pinterest.com/search/pins/?rs=ac&len=2&q=ceiling%20fan%20makeover&eq=ceiling%20fan&etslf=6079&term_meta[]=ceiling%7Cautocomplete%7C1&term_meta[]=fan%7Cautocomplete%7C1&term_meta[]=makeover%7Cautocomplete%7C1

AND

https://www.pinterest.com/search/pins/?rs=ac&len=2&q=ceiling%20fan%20repurposed%20decor&eq=ceiling%20fan%20repurposed&etslf=22147&term_meta[]=ceiling%7Cautocomplete%7C1&term_meta[]=fan%7Cautocomplete%7C1&term_meta[]=repurposed%7Cautocomplete%7C1&term_meta[]=decor%7Cautocomplete%7C1

I agree with spray paint. Silver or black could go along way for updating the feel.

https://www.hometalk.com/diy/floors-ceilings/s-13-ways-to-upgrade-your-boring-ceiling-fan-on-a-budget-18202730

Could also cover the blades with fabric.