> Yes, language/culture would be French.
I've learned it never pays to assume. :)
For the 14th-16th C, <Ysabel> is a fine choice - the 1292
census of Paris has <Ysabel>, <Ysabelon>, <Ysabiau>, <Ysabelot>,
and <Isabelot> , and tax rolls from Paris in 1421, 1423, and
1438 have <Isabel>, <Ysabeau>, <Ysabel>, and <Ysabelet>.  In
16th century Brittany, there's lots of diffferent spellings: 
Izabelle 1529, 1529
Ysabelle 1509, 1521
Yzabelle 1527, 1527, 1528, 1541, 1544, 1550
I haven't yet found a French example of <Marta> - though the
sources listed above have a similar name, <Martine>.
Source  has the surnames <Blanche>, <La Blanche>, and <Le
Blanc> (the first two are feminine forms, the latter masculine),
as well as <Bertran>. Source  has examples of <Bertran>,
<Bertrant>, and <Blanc> all as given names; since a common type
of byname in medieval France is the patronymic byname (where the
bearer uses her father's given name as her byname), any of these
three would also be fine choices for your byname.
And for the later part of your period, <Le Blanc> occurs in Paris
in 1571 (by this period, both men and women could have used this
form; in fact, all of the examples in this paragraph are of bynames
borne by women), <dite Le Blanc> 1581 'called the Blanc', and <Bertrand>
1547, 1577, 1612. 
So, some form of your name is going to be appropriate for pretty much
all the period you're interested in, though the spellings may have
changed some depending on the period.
vita sine literis mors est