Spotting

I've only seen brown and southern here in Georgia but I think you got a great point with the type of web. Widows' webs are tough and usually somewhat unorganized. With the lack of markings it could be female not yet turned all black.

The only Latrodectus species I know of that's in Washington state is the western black widow (Latrodectus hesperus), and I'd be sold on this being one if I could see the underside of the abdomen. Do you happen to have any other photos?

Well, I realize now it's Washington state and not DC:) I think Black Widow. Males and juveniles can be other than females that are jet black. Brown widows are making the creep north also but usually have

Looks like a widow to me. I think black widow but the range overlaps so it may be Northern or Southern.