As a preliminary matter, it is NEVER proper to write the required attire on the invitation itself. People will figure out what they are supposed to wear based on the formality of the invitation and by word of mouth. So whatever you choose to go with, you should be aware that it is an etiquette faux pas to write this on the invitation. If you feel you must include it, it's slightly more acceptable to write it on the reception card instead of on the invitation itself.
Additionally, if you write "black tie" on the invitations, this means that black tie is required, not optional (meaning men MUST wear tuxes and women must wear long gowns). If you want black tie to be optional, you need to put "black tie optional" or "formal dress." (Some etiquette books will tell you that black tie optional is also incorrect, but I think most people know what it means). At a black tie optional or formal event, men can wear a dark suit in lieu of a tux. Women can wear long dresses or fancy cocktail dresses.
Unless you run with a high society crowd who frequent black tie events, I would imagine your guests will be annoyed by a black tie mandate (especially if they think black tie is required). Most people don't have a tux lying around, so to attend a black tie event they would need to rent or buy one. This could cost anywhere from 75 to 200 dollars, depending on what you rent and where you rent from. Additionally, a lot of women may not have a long evening dress lying around, so they would need to purchase something as well (I can think of only two friends of mine who own formal wear, and both work in PR and attend gala events regularly). Add this to the cost of attending a wedding generally, and you might make attending your wedding cost prohibitive to some of your guests. At the end of the day, do you care that 100% of your guests are in tuxes and evening gowns but twenty people you care about can't come because they couldn't afford to pay for airfare, hotel, gift, AND a tuxedo and/or gown? Unless it's for a family member or a very close friend, I'd generally decline an invite to a black tie event because the added expense is more than I am willing to spend for DH and I to attend a wedding. I don't own an evening gown, so if DH and I had to attend a black tie event next week it would cost us at least $75 for the tux rental and maybe another $200 for a dress for me, in addition to whatever we're already spending on a gift and travel.
On the registry question - if you're not having a website, then word of mouth is the way to spread the word. People attending must know SOMEONE involved in the wedding, whether it's you, your bridal party, or one of your families, so they will find someone to ask. If there are people being invited who don't know anyone they could ask, then why would they be invited??