If an undead character is under the effect of a Mind Blank spell, does the spell's effect offer protection against being forced to cower or flee from a turn-undead effect?
No. Mind blank protects against:
- mind-affecting spells and effects (which are useless against undead anyways)
- information gathering by divination spells or effects (which obviously turn undead isn't)
Turn undead isn't either. However, I suggest you look at different items of turn resistance if that is what you are after.
The spell Life Ward (Cl 4) from the spell compendium makes you completely immune.