I put synthetic blend on a honda civic 237000 miles?

Honestly, without knowing much about oil, I bought a synthetic blend valvoline and put it on the honda civic like an idiot. Tbh, I have no idea why it has a lot of miles on a year 2000. But can anybody explain if this is a bad thing?

No, this is not a bad thing. Synthetic will work just as well if not better than convention oil. My advice would be to switch to a High Mileage conventional oil on your next oil change. Synthetic will provide no real benefit in your Honda.

Nope - that's a good choice… Or mistake. Full synthetic is somewhat better (lower pour point, better detergency) but blend is a good balance of cost and benefits of synthetic. If conventional oil has been used regularly the blend won't be as aggressive about adsorbing deposits and if synthetic has been used regularly the blend is a very small change.

Better to be lucky than good! It's the motto I live by.

Don't be so hard on yourself. No harm done. Just wait until the next oil change. If you like to enrich your knowledge about oil, this article is very helpful:
http://www.carbibles.com/engineoil_bible.html

Synthetic oil is the best there's for cars. You may have bought semi synthetic which is still fine. But fully synthetic is better in all respects. You should use the viscosity as recommended by Honda. But even here there's leeway. Asda/Wal-Mart is the place to buy it.