Why coastal blackouts mattered against U-boats

Would earlier US experience navigating off blacked-out Britain have improved the response to U-boats off the American coast?

No; the main issue was not navigation but civilian lighting that made ships visible in silhouette to attacking submarines.

Experience navigating near a darkened British coast twenty years earlier would not have fixed the American problem. Sailors already had to navigate without visible land; the decisive failure was leaving shore lights on, which silhouetted vessels for U-boats operating off the US East Coast.