The dark color of the
oil is most likely from assembly lube. I had the same black greasy magnetic stuff on my plug after the first couple of changes. I wouldn't worry to much about that on a new engine. The most likely failure is a bearing and they aren't magnetic. If you see a gold or bronze sheen in the
oil then worry. Imagine 24 new rings sliding up and down in 8 new bores for 500 miles, you got to get some wear.