in TeamCity.ServiceMessages/Write/Special/Impl/Writer/TeamCityBlockWriter.cs [37:44]
public TCloseBlock OpenBlock(string blockName)
{
if (blockName == null) throw new ArgumentNullException(nameof(blockName));
AssertNoChildOpened();
PostMessage(new ServiceMessage("blockOpened") {{"name", blockName}});
_isChildOpenned++;
return _closeBlock(new DisposableDelegate(() => CloseBlock(blockName)));
}