The portal spawning algorithms have gone though a lot of changes to get to the point they are now, and it used to be the case that you'd end up miles from your base when this happened but Notch linked them properly so hopping back in your Nether portal should take you right back to where you came from. As with everything Minecraft, this is not a guarantee however!
As StrixVaria said, carrying a compass is vital to helping you get back from any expedition but this does point to your spawn, not your base.
The short answer is no. When You enter the a portal from the nether, on your way to the overworld, Minecraft calculates the primary portal coordinates with this generalized equation:
{X, Y, Z} → {floor(X) × 8, Y, floor(Z) × 8}
The game then checks for an active portal in a 128 block radius around that location. Given your nether portal coordinates:
X: -70; Y: 72; Z: -139
Your overworld portal must be within the following horizontal bounds:
X = -688, -432
Z = -1240, -984
Remember, any overworld portals within 1024 blocks of each other will link to the same nether portal, because 1024 blocks in the overworld = 128 blocks in the Nether, and the game checks for portals in a radius of 128 blocks.
If you build a new nether portal at the blaze farm and destroy your old portal, your main base portal will probably link up to your blaze farm. However, when you attempt to go back to the overworld, Minecraft will look for a portal within the above bounds and create a new one if it doesn't find one.
Your best option is probably to build a rail line from your current portal to the blaze farm. Ghasts can destroy any block with a blast resistance below 20.17, but they won't shoot at you without a line of sight, so you can make an inexpensive safety-tunnel around your rail line with pure Netherrack.
You can also build a more scenic tunnel with stone, glass, leaves, fences, etc, since Ghasts cannot "see" through transparent blocks.
Best Answer
Note: This answer applies to Minecraft 1.4.1 and earlier. For newer versions see the other answers.
Mobs can not follow you through a portal. "Follow" meaning continuing a pursuit by entering the portal after you do and chasing you again once you're on the other side.
Currently, the only danger coming from the existence of a portal is the possibility of a Ghast spawning by it in the "Earth", "Overworld" or whatever we now call the main area of the game. The space they need to spawn there is at least 4x4x4 and, according to Notch, their appearance near the portal is "extremely uncommon".