From the official BlackLab site: "BlackLab is a corpus retrieval engine built on top of Apache Lucene. It allows fast, complex searches with accurate hit highlighting on large, tagged and annotated, bodies of text. It was developed at the Institute of Dutch Lexicology (INL) to provide a fast and feature-rich search interface on our historical and contemporary text corpora.

BlackLab is licensed under the Apache License 2.0."

BlackLab is not an end-user application, but an extension to Apache Lucene.  It requires programming experience to use.  Several sites have so far been built using BlackLab.  The BlackLab FAQ at https://github.com/INL/BlackLab/wiki/Frequently-Asked-Questions lists several of these.

BlackLab is very useful as search engine for morphologically adorned corpora.  The principal search language syntax is similar to that of traditional corpus search engine tools such as the Corpus Workbench or the Sketch Engine.