Can I seal indian sandstones still apparentwith some lichen

I would strongly suggest you use Lithofin products and ask their advice, they have a helpline. You can't really damage the stone, discolour yes, but damage no, unless its really cheap and not suitable for paving. There are a lot of different sealants, so thats a hard question to answer. Test it first is the general rule. If you've got black spot you'll get it again, the paving is probably cheap and too porous, therefore holding water or laid too flat. Basically its too wet.