plugin documentation suggestions:

[gregcaporaso]

threads parameter should be a range bound on the lower end. Also, note that this is only relevant for vsearch?

https://github.com/nbokulich/q2-quality-control/blob/6d89fbebe9557fde5c7a1ffef1c542564d8e551d/q2_quality_control/plugin_setup.py#L38

The following:
https://github.com/nbokulich/q2-quality-control/blob/6d89fbebe9557fde5c7a1ffef1c542564d8e551d/q2_quality_control/plugin_setup.py#L50

would be more clear as: BLAST expectation (E) value threshold

This description should mention the E-value threshold too, and how that interacts with percent identity (i.e., do both need to be above their respective thresholds)? Sort of related, but should it be possible to disable the percent identity filter, and only filter based on E-value?

https://github.com/nbokulich/q2-quality-control/blob/6d89fbebe9557fde5c7a1ffef1c542564d8e551d/q2_quality_control/plugin_setup.py#L64

[nbokulich]

e value and perc_identity are both used for filtering, and hence to be accepted as a hit a sequence must have a lower E and higher identity than the respective thresholds. This is now noted in the description.

Default E val is now set to the default used in blastn.