It's not that bad...
With no range listed, the the prestige class blighter's supernatural ability blightfire has a range of 0 ft., therefore centered on a crosshairs adjacent to his space. This means the blighter will be affected by his own blightfire effect.1
But, assuming no shenanigans, the blightfire's effect deals only an average of 17.5 points of fire damage and a maximum of 30 points of fire damage. "The blighter’s caster level is equal to her blighter level plus her druid level" (Complete Divine 23), and the path of least resistance for blighter entry is as a level 6 druid. Thus By the time the blighter can use the special ability blightfire, the blighter should (assuming the blighter possesses a Wisdom score of at least 14) have access to the 2nd-level blighter spell resist elements, which doesn't exist2 yet is almost certainly supposed to be, instead, resist energy [abjur] (PH 272).
Such a blighter, then, can cast resist energy at caster level 8, providing the blighter and his equipment (as per the spell's description) fire resistance 20 for 80 minutes per casting, which should be sufficient to ward himself from his own effect except on an exceptionally lucky—or unlucky—roll. At caster level 11 (druid 6/blighter 5), the spell resist energy renders the blighter immune to his own blightfire ability.
...But it's also not that good
Being able to take a standard action to deal 5d6 points of fire damage with a 0-ft.-range in a 10-ft.-radius burst as a level 8 character just isn't that impressive, even if such an effect can be generated at will. The special ability blightfire never increases and is already behind the damage curve of most damaging effects, involves forcing many a blighter to protect himself from his own special ability, and hurts equally any friends the blighter may have (and those friends' equipment).
The special ability blightfire is a little sad, actually, especially compared to what the blighter could have been doing had he just chosen to be, from the start, an evil druid.
1 When the blighter appeared in the Dungeons and Dragons, Third Edition supplement Masters of the Wild the supernatural ability blightfire was like the 1st-level Sor/Wiz spell burning hands [evoc] (PH 207), the effect of which can also set certain objects alight. A DM may want to house rule the special ability blightfire as functioning similarly instead with its weird 0-ft. range.
2 The blighter's spell list appears copied from its Dungeons and Dragons, Third Edition source in which the spell resist elements did appear and where it was quite different from the spell resist energy.