Butchers Block worktop going black around sink?

This article describes 11 different types of varnish...
https://en.wikipedia.org/wiki/Varnish

It might help you decide what to use...

I also found this...
Kitchen counter tops of natural wood pose special finishing problems. There are reasons for the popularity of melamine! The surface must be perfectly sealed against moisture to avoid staining by food and to avoid buildup of moisture that would permit toxic mould growth. The finish has to be sufficiently tough to withstand daily scrubbing, and must be continuously renewed whenever it gets too thin. But, most importantly, oils that could support microorganism growth must be totally prevented from soaking in. If you prepare raw chicken on a salad bowl type finish, you are risking your life.

Polymerised tung oil works! Simply renew the finish often enough that the surface stays perfectly sealed - a once-a-month rub with an oil-soaked cloth is about right, with overnight to dry. Tung is excellent for this because it bonds naturally to old layers, unlike urethanes and most other synthetic finishes.

Good Luck...
February 2016