so you need scrapebox to find the expired domains you can use a custom footprint like sit:tumblr.com and then use around 40 keywords relating to your money making blog star harvesting, run the harvest through vanity name checker the check the URL authority disregard the low authority URLs then attempt to register them to your Tumblr account.
as for Alexa, i don't really ever check someone else may have more knowledge on that.