Equity portfolios can't help being global. Multinationals account for most stock market assets, and they sell their wares everywhere. Where a blue-chip company is headquartered does not indicate its revenue sources.
According to Morningstar's latest calculations, 62% of revenues for S&P 500 companies come from the United States. As S&P 500 firms account for 80% of the capitalisation of the Wilshire 5000 - which reflects the majority of U.S. traded stocks – that index scores similarly. Consequently, shareholders of major U.S. stock market index funds have about 40% foreign exposure, as measured by corporate revenues.
Unsurprisingly, given that the U.S. is easily the largest consumer market, this figure is above other countries'. Japan and Australia are close, with 59% of the Nikkei 400's revenues arriving from inside Japan, and 58% of the S&P/ASX 200's revenues being from Oz. However, the United Kingdom's amount is far lower, with domestic revenues of 22%.