Can't install newest Blast from Conda
I was able to reproduce your problem, even in a blank environment.

$ conda create -n test
$ conda activate test
$ conda install bioconda::blast=2.7.1
Solving environment: failed

PackagesNotFoundError: The following packages are not available from current channels:

  - bioconda::blast=2.7.1
  - boost=1.64
  - bioconda::blast=2.7.1
  - gnutls
  - bioconda::blast=2.7.1
  - nettle[version='3.3|3.3.*']
  - bioconda::blast=2.7.1


Note that boost, gnutls, and nettle are listed as requirements here.

If you do a search for those packages, you'll see that boost is available in a default channel, whereas gnutls and nettle are only in the conda-forge channel, which is not a default.
Adding those packages to the install command resolved this issue, for me.

$ conda install bioconda::blast=2.7.1 conda-forge::gnutls conda-forge::nettle --dry-run
Solving environment: done

## Package Plan ##

environment location: /Users/hawleyj/anaconda3/envs/test

added / updated specs: 
    - bioconda::blast=2.7.1
    - conda-forge::gnutls
    - conda-forge::nettle
...


As mentioned in the comment below, to avoid this issue entirely, before using Bioconda packages make sure you run the proper channel configuration first.

conda config --add channels defaults
conda config --add channels bioconda
conda config --add channels conda-forge