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Your technique is the most robust. Its the one I use myself. At times content may be delivered over HTTPS and the use of about:blank can cause warning messages to appear to the effect of "do you want to include content from unsecure location" or some such thing.
Something being instant is a matter of perception however if you have a single Blank.html file on your site configured with a long cache expiry the client will only ever fetch the page once (at the most once per-session).
I have had difficulties with "about:blank" on pages with many IFrames. It does not seem to be a valid location in every browser (I never found out for sure, though). Anyway, I am happy with javascript:void(0);