Welcome stone seller.
Blaen y Cae, mmm.
Blaen can have a number of meaning depending on the context. In its most simple form it means foremost and may be compared with cyntaf or arweiniol.
When combined with say, pen,(pen=, chief, head,end,top) it can mutates to penmain/penmaen and means point or tip (as of nose, not rubbish!). Hence Penmaenmawr (mawr means big/great).
Blaen afon means the upper reaches of a/the river (afon is a river) and Blaen y nant is the upper reaches of a brook.
Cae is a field.
The "foremost field" might seem quite a reasonable translation, as might " the field of upper reaches"
I'm sure others will join in, if only to correct me!
Hope this make sense and helps!
What stone do you sell?
PS You may find www.geiriadur.net of use. It's the on-line dictionary of the Univesity of Wales, Lampeter. Also, be careful, because you may find that you are dealing with 19th century Welsh which can be somewhat different to modern Welsh.