Get a list of up to 20K tumblr users from tumblr search based on your keywords/tags. You can follow the list of people and maybe they will follow you back. (I have another gig that can do the following for you.) if you use the list for this purpose, the list is enough for 100 days of following because tumblr has a daily limit of 200.
For niche or keyword oriented marketing on Tumblr, this is what you can do: search on Tumblr home page by tags and you will get all blogs/users that are related to (or interested in) your niche.
How it’s done: I search by tags on Tumblr homepage, go through the listed results one by one, and save the users in a text file. The file will not contain duplicate users.
You will need to give me your keywords separated by commas. You need to arrange them by significance because I might never use the keywords in the latter part of your keyword list. I will stop as soon as I gather 20K users. How many keywords I consume depends on the tumblr search results.