In my experience, blending herbs that have different ascribed magical
properties is commonly done to create new recipes for use in
spell-work, and in some cases the blending does seem to alter the
qualities of the herbs in those mixtures. However, before taking that
as an unconditional "yes" answer (which it is not), also be aware of
three further factors:

1) Magical properties of herbs do vary by culture sometimes, so what
you see on one site may not be mentioned at another site. Examples:
  Ginseng: Asian = male vigour; African-American: gambling luck
  Yarrow: European = courage and wound-healing;  Chinese: divination
  Yellow Dock Root: African-American = money; Iroquois = love-doll

2) Magical properties of some herbs contain their own opposites, most
notably herbs used both to banish/curse and protect/unjinx. In these
cases, often the WAY the herb is used factors into the kind of spell
it appears in. Example:
  Vandal Root: can be used to jinx, to protect, and for coercive love

3) Some herb sites on the web are not reliable, just as some books are
not reliable. If authenticity of magical ascriptions is important to
you, check and compare several sources. When evaluating sources, if
you have doubts, you can try a few work-arounds like  giving
preference to sources that also list the taxonomic (Latin) name of the
herbs and/or giving preference to sites that list a bibliography of
sources, because these two factors imply a basic level of research on
the part of the author. This is not to say that an illiterate
herbalist will not have great knowledge -- but web authors, by their
very nature, are not illiterate, so they can be tentatively evaluated
by the usual standards for scholarship.

Basil has a long history in Europe of protection with respect to the
family home. It is not generally used to protect a business. In India
it is also considered productive of family and marital happiness. The
saying "Evil cannot come where Basil has been" can be found in old
European herb books and has transferred into the African-American
community in the use of dried basil in floor-sweeps (sprinkle on the
floor, sweep out the door) and as an ingredient in herb-teas brewed
for use as a floor wash (often with the addition of Rosemary and Rue).

Sage is quite good for cleansing. As Brenda noted, there is a
difference between cleansing a house (e.g. with Sage), protecting it,
and reversing or returning curses and jinxes already sent.

I personally do the first two, with the third optional if i believe i
am under attack. I do them in the same order that Brenda described:
First clean the house out, then protect it else the evil will be
trapped inside, then (optionally) reverse all future incoming evil to
its sender.

Other herbs i use for cleansing: Lemon Grass, Camphor (in the form of
squares of resin crumbled burned as incense on charcoal), etc.

Other herbs i use for protection: Red pepper, Salt (not an herb per
se, but used as such in the hoodoo tradition), Black Pepper, Camphor
(in the form of whole squares of resin placed in the corners of a room
or building), Anise, Boldo, Devil Pod (Bat Nut), Devil's Shoe Strings,
etc.

Herbs i use to reverse, return, or send back evil: Agrimony,
Eucalyptus, Devil Pod (Bat Nut),  Devil's Shoe Strings, etc. The use
of mirrors for reversing incoming attacks is also common, both in
Chinese feng shui and in African-American hoodoo.  

Here are two web pages that may be of interest to you as you research herbs:

hoodoo herb and root magic:
http://www.luckymojo.com/hoodooherbmagic.html
-- This is a link to information about a book on herb magic that i
have written. The book is for sale, and this is a commercial page, but
on the page there are also 6 complete sample entries (out of 500 in
the book) to read for free.

hoodoo at a glance (herb list)
http://www.luckymojo.com/hoodooataglance.html
-- This is a web page that was compiled and given to my herb store as
a gift by one of our customers. It lists many common herbs, minerals,
and formulas for oils that we sell in the shop, with ascriptions for
use, but you need not purchase the herbs from us to use the page. The
list is not complete, but it gives a good overview of about 100 herbs,
sorted by condition (e.g. love, money, protection, etc.).

FLAME ALERT: This post was made in serious reply to two posters who
are known to engage in flame wars. Replies posted to this message from
these and other trolls in arw, aw, and ap may be vicious and
off-topic. This post is my first test of a new kill-file i am working
on, to see if it can be enjoyable for me to post substantively to arw
and other pagan newsgroups. See the alt.religion.wicca troll roster at
     http://www.luckymojo.com/faqs/altreligionwiccatrolls.html
for details. Flames will not be seen by me, so don't bother.  

Cordially,

cat yronwode