Is that a way to custom blockselect style?

Webix JS
1

for example: http://handsontable.com/demo/custom_borders.html

2

You can’t set custom border color, but you can set a custom background|text color

http://webix.com/snippet/9460abbf

In webix datatable the blockselection just marks the cells as selected, it doesn’t create any custom area object which can be styled separately.