I'm not really expert in the cost of magic items, but i can say it is not as cheap as 100gp.

First, the spell you are looking after is more of the kind of Faerie Fire or Unwelcome Halo, both lvl 1.

Second, to enable it at the same time of your Blood Reservoir you can use the same command word. The cost of such item is 1800 gp * caster level * spell level = 1800 * 1 * 1 gp = 1800 gp (see [Magic Item Creation](http://paizo.com/pathfinderRPG/prd/magicItems/magicItemCreation.html), under _Magic Item Gold Piece Values > Table: Estimating Magic Item Gold Piece Values > Spell Effect > Command word: Spell level × caster level × 1,800 gp_) very close to the Blood Reservoir. You just need caster lvl 1 because those spells last 1 minute/level. Suppose they modify the spell to works like a lvl 0 spell, then the spell multiplier for costs is 0.5, dropping the item cost to 900 gp; limit it to some rounds, maybe just 5.

Limiting the usages per day could drop the cost more:

 - 1 per day = 900 gp / 5 = 180 gp
 - 2 per day = 900 gp / 2.5 = 360 gp
 - 3 per day = 900 gp / 1.67 = 540 gp
 - 4 per day = 900 gp / 1.25 = 720 gp
 - 5 per day = 900 gp / 1 = 900 gp

To say the item is just cosmetic is to underestimate the item; and, sorry to say it, a lie. It is powerful enough. You clearly stated you want it to be able to bluff foes. In addition: It can be detected by magic, so friends can find you using detect magic; anyway they can find you if you shed light where in darkness and some other concealing conditions, like fog ("Hey guys, just follow me!"); useful to send signals; it can make you momentarily visible when invisible without breaking the invisibility; it can ignore concealment (even provided by blur); among other creative uses (i can imagine i use it to bluff the orc boss "boss, take it, it is a powerful item that brings you the strength of a bull, just cast the word and you're done" then we fight them in the middle of the dark, he uses the item, we all see him and we all use our ranged weapon to take him down).