Yes, you can have a white lamb from two black Hebs. That's how our white line started. And we were told the same thing: that a white ram must've have gotten to the ewe - except it didn't. The black gene is actually recessive in sheep, and Hebs are predominantly black because they were selectively bred for that colour. Other colours were culled. But there is an occasional throwback. We now have a full line of whites, with good records of genealogy - the ewe who started it is gone, but her daughters now carry the line, giving us both white and black lambs, sometimes twins of each. We had our first white male last year, it will be interesting to see what happens when we breed him.
We have russets (raddies), silverbacks/greys, and piebalds too. We were led to believe grey was a sign of age or nutrition...and this may be true - the highlands have issues with poor grazing. But we have a healthy fat ewe who has been silvering since her first sheering. And these colours were spoke of since the sheep were known, and if they're affected by their environment, no surprise. Some of our best sheep are silver. And russet, which was said to be from sun bleaching. In fact, any colour other than black was seen for a long time as being because of something outside of the sheep's nature. Something gone wrong.
Farms like ours, while admiring and recognizing the black modern version of the Heb, are working to bring recognition to the multi-coloured heritage of the breed. I'd be curious to know where your ram came from, and what your white lamb does, when bred.
You will get told it's not pure Heb. All I can recommend is to keep good records - and fences - if you want to be able to stand your ground on this issue.