The list of touch-range, damage-dealing spells of level 5 and below that aren't on the Duskblade list already isn't very long. At least not if you further restrict that list to "useful spells".

Anyway, there are a few well-known ways to get extra spells known/on to a general class list (there are no Duskblade-specific methods) that can be useful here:

1. The Recaster (Races of Eberron) PrC can grant any spell at all.
2. Wyrm Wizard (Dragon Magic) can grant any spell at all.
3. The Extra Spell feat (Complete Arcane) can grant any spell at all - if it works at all. That latter point is often debated.
4. Domains grant spells known. The most general, easily accessible way to get some Domain access on an arcane spellcasting class is probably the Arcane Disciple feat (Complete Divine), although that carries heavy limitations. Depending on the specific domain, you may be able to get access through a PrC.

Now, then there is the question of what spells you actually want.

• Combust (Spell Compendium) is a rather good spell, scales well with metamagic. Methods: 1, 2 and 3.
• Wracking touch (Spell Compendium) is OK if you have lots of Sneak Attack damage somehow. Methods: 1, 2 and 3.
• Belker Claws (Spell Compendium) are rather bad but cool. Methods: 1, 2 and 3.
• Bestow Wound (Heroes of Horror) is a cool but kind of expensive way to heal. Methods: 1, 2, 3 and 4 (Spite domain).
• The Inflict X Wounds line of spells is bad. You can get a few of them via method 4 (Destruction domain, probably others). There are at least a few more that qualify (like Parching Touch from Sandstorm), but I don't know of more worth mentioning.

Harm would be great here, but I don't know of a way to get it as a 5th level spell.

But, at the bottom line... I don't think it's worth the opportunity cost in any of these cases. Maybe for Combust, but Duskblades are rather build-constrained as it is.

A cleric cannot cast spells with an opposed alignment descriptor

Taken from the Cleric description in 3.5 SRD - similar rule appears in PHB:

Chaotic, Evil, Good, and Lawful Spells

A cleric can’t cast spells of an alignment opposed to his own or his deity’s (if he has one). Spells associated with particular alignments are indicated by the chaos, evil, good, and law descriptors in their spell descriptions.

So as an evil cleric, or a cleric of an evil deity, your character is forbidden from using any [Good] spells.

I'm not aware of any rule prohibiting an evil cleric to cast spells with the [Light] descriptor.

Regarding spell descriptors...

I recommend you read the SRD text about spell descriptors. It isn't very long, and says (among other things):

Most of these descriptors have no game effect by themselves, but they govern how the spell interacts with other spells, with special abilities, with unusual creatures, with alignment, and so on.

For example, spells with the [Good] descriptor have special effect on certain evil outsiders, can't be cast be evil clerics, and can be detected using the Detect Evil spell. Similarly, some creatures (mostly undead and darkness-themed) are more vulnerable to [Light] spells.