As of till now there is a few other blockchain based blogging sites but all of them are specific demographic based. For example there is golos.io, an exact copy of steemit but based on russian linguistics and the majority of its community are also russian. It has golos, a token works similarly as steem and GP similar to Steem Power.
Another one would be scorum.com . Pretty similar to steemit but its content are sports driven. Both golos and scorum uses a DPOS based blockchain to validate its blocks . Other ones like mythril or uno also works with similar blockchains
Now to come to the real question, are they steems competetion? When you say steem as a boockchain based blogging platform you know that steem supports a variety of content typology where other ones are specific content driven and demographic based. For example, scorum is a sports blogging site, golos is russian, lit is photography based lifestyle blogging platform. To take them as comepetition they are but only partially. Whereas steem is a self sufficient and is whole on its own. So to me i think they are somewhat of a competition but to surpass steem they have a long way to go.
And another thing is Dan Larimar spoke about bringing steem 2.0 based on eos. Untill that there are no true cometition to steem as of now. Atleast thats what i think.