A feature of Tumblr is to allow others to post on your blog (although not many blogs have this feature turned on).
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 that are related to your niche. You can submit your posts to these related blogs, and get more exposure for your posts.
However, it is up to you and the blog owner to have your posts posted on his/her blog. I cannot guarantee acceptance of your posts. I just find the blogs for you.
For this gig, I will give you up to 60 blogs that you can submit posts to based on the keywords you provide.
How I do it: I search by tags on Tumblr homepage, go through the found blogs one by one to see if they accept post submission. I will give you the list of url in a text file.
You will need to give me your keywords separated by commas. You need to arrange them by significance. I will go through the keywords until I find the 60 urls.
**Note that you must have a blog and login in your blog, in order to submit.
You can order unlimited amount of this gig.